[BUG] linux-cachyos 7.2.2: RX 7900 XTX hard-freezes on DPMS off at 1440p 360 Hz; Arch 7.1 and LTS work

### Pre-flight checklist

- [x] I have searched existing issues and this is not a duplicate.
- [x] I have read the [Contributing Guidelines](https://github.com/CachyOS/linux-cachyos/blob/master/CONTRIBUTING.md).
- [x] I have verified the issue is reproducible with the latest available CachyOS kernel.
- [x] I have tried to reproduce the issue on Arch Linux's `linux` kernel.

### Upstream / vanilla kernel check

I tested with a vanilla/upstream kernel and the issue does NOT reproduce there (CachyOS-specific bug)

### Kernel variant

linux-cachyos (EEVDF, Clang)

### System information (cachyos-bugreport.sh)

```text
https://paste.cachyos.org/p/cd99097.log
```

### Manual system information (if cachyos-bugreport.sh is unavailable)

```text

```

### Bug description

On linux-cachyos 7.2.2-1-cachyos, the system hard-locks reproducibly when DPMS turns off a Samsung Odyssey OLED G6 G60SD running at 2560x1440 @ 360 Hz over DisplayPort on an RX 7900 XTX.

The same kernel and monitor work correctly at 240 Hz, including with VRR enabled. The monitor also works correctly at 360 Hz on linux-cachyos-lts 6.18.42 and on Arch's vanilla linux 7.1.11.arch1-1. Disabling VRR, testing amdgpu.runpm=0, and connecting the keyboard directly to the PC do not prevent the lockup on linux-cachyos 7.2.2.

This appears to be a CachyOS-kernel-specific regression in the newer AMD Display Core path triggered by the 360 Hz high-bandwidth/DSC mode. Current workaround: use 240 Hz with VRR enabled, or boot the LTS/Arch kernel.

### Steps to reproduce

1. Boot CachyOS with linux-cachyos 7.2.2-1-cachyos on the RX 7900 XTX.
2. Set the Samsung G60SD on DP-1 to 2560x1440 @ 360 Hz (mode 89):
kscreen-doctor output.DP-1.mode.89
3. VRR may be Automatic or Never; the result is the same.
4. Turn off all displays through DPMS:
kscreen-doctor --dpms off
5. Wait approximately 10 seconds and try to wake the displays.

The freeze reproduces every time at 360 Hz on linux-cachyos 7.2.2. Repeating the same sequence at 240 Hz works. At 360 Hz it also works on Arch linux 7.1.11.arch1-1 and linux-cachyos-lts 6.18.42.

### Expected behavior

The displays should enter DPMS off without affecting the running system and should wake normally from keyboard or mouse input.

### Actual behavior

The keyboard and mouse lighting initially turns off. After roughly 10 seconds it comes back on and a fan briefly speeds up, but the PC is then completely unresponsive and no display signal returns. Keyboard/mouse wake does not work. The reset button appears to restart the machine but still produces no picture. A power-off followed by a cold start is required.

### Logs / stack traces

```text
Full redacted cachyos-bugreport output:
https://paste.cachyos.org/p/cd99097.log

The journal from a failing boot ends abruptly at the lockup and no pstore record is produced, so there is no stack trace.

AMD Display Core comparison on the same RX 7900 XTX with the same DMUB firmware 0x07003300:
- linux-cachyos 7.2.2: DC 3.2.384 (fails at 360 Hz)
- Arch linux 7.1.11: DC 3.2.378 (works at 360 Hz)
- linux-cachyos-lts 6.18.42: DC 3.2.351 (works at 360 Hz)
```

### Additional system information

Hardware:
- AMD Ryzen 9 7900X
- Sapphire NITRO+ Radeon RX 7900 XTX (Navi 31), amdgpu
- ASUS TUF GAMING X670E-PLUS, BIOS 3881 (2026-06-17)
- Samsung Odyssey OLED G6 G60SD, 2560x1440, DisplayPort
- Two Dell S2721DS displays at 2560x1440 @ 75 Hz

Desktop/software:
- KDE Plasma 6.7.4, Wayland, KWin 6.7.4
- Mesa 26.2.1
- libdrm 2.4.134
- linux-firmware-amdgpu 20260810-2

A/B results:
| Kernel | Samsung mode | VRR | Result |
|---|---:|---|---|
| linux-cachyos 7.2.2 | 360 Hz | Automatic | hard lock |
| linux-cachyos 7.2.2 | 360 Hz | Never | hard lock |
| linux-cachyos 7.2.2 + amdgpu.runpm=0 | 360 Hz | Never | hard lock |
| linux-cachyos 7.2.2 | 240 Hz | Automatic | works |
| linux-cachyos 7.2.2 | 240 Hz | Never | works |
| Arch linux 7.1.11.arch1-1 | 360 Hz | tested | works |
| linux-cachyos-lts 6.18.42 | 360 Hz | tested | works |

The 360 Hz EDID mode has a 1667.17 MHz pixel clock; the 240 Hz mode has 1111.25 MHz. The display's FreeSync range advertises a maximum of 240 Hz, but the failure also occurs with VRR disabled.

Possibly related but with a different trigger/hardware: #1007.