[BUG] RX 6600 + LG OLED (HDMI 2.1): 4K border/overscan and HDR color regression, 7.1.6 → 7.2.0

### 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/f34aafb.log
```

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

```text

```

### Bug description

## System
- GPU: AMD RX 6600
- Display: LG OLED CX 65" (model OLED65CXPUA), connected via HDMI 2.1
- Distro: CachyOS
- Desktop: KDE Plasma 6.7.4

## Description
With v7.2.0-1, At 4K (3840x2160), the TV shows an incorrect overscan/border that is not
present at 1080p. Additionally, when HDR is enabled at 4K, colors are
shifted/incorrect on top of the border issue. 1080p is unaffected in
both SDR and HDR. 4K SDR has the border issue only (no color shift).
4K HDR has both the border issue and the color shift.

Confirmed via bisection that this is NOT related to KWin, Mesa/RADV,
or amdgpu firmware — all were individually downgraded to their prior
versions with no change in behavior. Only downgrading the
`linux-cachyos` kernel package resolved the issue.

## Regression window
- Last known good: `linux-cachyos` 7.1.6-1
- First known bad: `linux-cachyos` 7.2.0-1

## Vanilla kernel test
Tested on Arch Linux's `linux` package, version 7.1.9-arch1-2 — issue
does NOT reproduce. Border and color are correct at 4K SDR and HDR.

## Bisection summary
| Component | Version tested | Result |
|---|---|---|
| kwin | downgraded 6.7.4-7.1 → 6.7.4-1.1 | no change |
| mesa/vulkan-radeon (+lib32) | downgraded 26.2.1-1 → 26.1.6-1 | no change |
| linux-firmware-amdgpu | downgraded 20260810-2 → 20260622-1 | no change |
| linux-cachyos | downgraded 7.2.0-1 → 7.1.6-1 | **fixed** |
| linux (vanilla Arch) | 7.1.9-arch1-2 | not reproducible |

### Steps to reproduce

1. Connect RX 6600 to LG CX OLED via HDMI 2.1, set resolution to
3840x2160
2. Observe overscan border at edges of screen (not present at 1080p)
3. Enable HDR — observe additional color shift on top of the border
issue

### Expected behavior

4K output (SDR and HDR) should display with correct scaling and no
overscan border, and colors should be accurate in HDR

### Actual behavior

See description

### Logs / stack traces

```text

```

### Additional system information

_No response_