[BUG] 7.1.6 + i915/Kaby Lake: plymouth-quit times out, holds DRM master -> kwin_wayland "Device or resource busy" White CachyOS logo; LTS 6.18.42 unaffected. Workaround: `plymouth.enable=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 have not tested with a vanilla/upstream kernel

### Kernel variant

linux-cachyos (EEVDF, Clang)

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

```text
My report was longer than the allowed character limit so here is the paste:

https://paste.cachyos.org/p/ef38f58.log
```

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

```text

```

### Bug description

After updating to `linux-cachyos 7.1.6-1`, the regular kernel no longer reaches a usable graphical session. Booting the same installation with `linux-cachyos-lts 6.18.42-1` works normally. No kernel/userspace configuration changes are involved; only the kernel differs.

Timeline of a failed boot (`linux-cachyos 7.1.6-1`):

1. The boot appears to hang for after the LUKS password prompt
2. After the LUKS volume is unlocked, `plymouth-quit.service` **times out** — plymouthd never quits and keeps DRM master on `/dev/dri/card1`.
3. I'm left looking at the white CachyOS logo

### Steps to reproduce

1. Fully update CachyOS (`linux-cachyos 7.1.6-1`, `linux-cachyos-lts 6.18.42-1`).
2. Reboot and select the normal `linux-cachyos` entry in Limine.
3. Unlock the LUKS volume, wait through switch-root.
4. I get the blue CachyOS logo, it goes to the white one, then hangs.
5. Reboot into `linux-cachyos-lts 6.18.42-1`: boot and desktop work normally.

### Expected behavior

The Plasma login manager should start and allow a normal KDE Plasma Wayland session on the `linux-cachyos` kernel, as it does on the LTS kernel.

### Actual behavior

I get the blue CachyOS logo, it goes to the white one, then hangs.

### Logs / stack traces

```text
Aug 06 13:09:52 systemd-cryptsetup[179]: Set cipher aes, mode xts-plain64, key size 512 bits for device /dev/disk/by-uuid/6ee64c27-8cb7-4caa-ac28-d7ce38e57622
Aug 06 13:10:20 systemd[1]: plymouth-quit.service: start operation timed out. Terminating.
Aug 06 13:10:20 systemd[1]: plymouth-quit.service: Failed with result 'timeout'.
Aug 06 13:10:20 systemd[1]: Failed to start Terminate Plymouth Boot Screen.
Aug 06 13:10:21 kwin_wayland[1206]: No backend specified, automatically choosing drm
Aug 06 13:10:21 kwin_wayland[1206]: Failed to open /dev/dri/card1 device (Device or resource busy)
... (repeats indefinitely)
Aug 06 13:10:26 plasma-login-greeter[1215]: Failed to create wl_display (Connection refused)
Aug 06 13:10:26 plasma-login-greeter[1215]: This application failed to start because no Qt platform plugin could be initialized.

For comparison, on the working LTS boot:

Aug 06 14:43:59 systemd[1]: Starting Terminate Plymouth Boot Screen...
Aug 06 14:43:59 systemd[1]: Finished Terminate Plymouth Boot Screen.
```

### Additional system information

### Workaround (confirmed)
Adding `plymouth.enable=0` to the `linux-cachyos` kernel command line (scoped via `KERNEL_CMDLINE[linux-cachyos]` in `/etc/default/limine`, then `limine-mkinitcpio`) makes `7.1.6-1` boot to a fully working desktop. The boot splash is skipped and the LUKS prompt appears on the text console.

### Additional system information
- Failing kernel: `linux-cachyos 7.1.6-1`; working: `linux-cachyos-lts 6.18.42-1`; userspace (Plasma/KWin/Mesa/plymouth/login-manager) is identical in both.
- Intel i915 only (no NVIDIA/AMD GPU in this machine), which shows this is **not** GPU-vendor-specific.
- Closely related reports: [#940](https://github.com/CachyOS/linux-cachyos/issues/940) (same `Device or resource busy` chain on NVIDIA, same plymouth workaround), [#959](https://github.com/CachyOS/linux-cachyos/issues/959) (7.1.6 login failure, LTS 6.18.42 works, amdgpu), and [CachyOS/distribution#346](https://github.com/CachyOS/distribution/issues/346) (general plymouth boot-screen hang, vendor-independent).