[BUG] RADV 26.2.1 regression on VanGogh (Steam Deck LCD): pixelated textures/shimmering under gamescope, fixed by downgrading to 26.1.6

### 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 ALSO reproduces there (upstream bug — please report upstream)

### Kernel variant

linux-cachyos-deckify

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

```text
CachyOS (deckify kernel 7.1.8), Steam Deck LCD (VanGogh, radv), gamescope session, Proton (proton-cachyos-slr) via Heroic/umu
```

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

```text

```

### Bug description

After upgrading mesa/vulkan-radeon/lib32-mesa/lib32-vulkan-radeon from 26.1.6 → 26.2.1 (Aug 21), D3D12 games render with blocky/pixelated textures and text plus heavy specular shimmering in game mode (gamescope WSI). Same game + same config renders correctly on desktop mode, and fully correct again after downgrading the entire mesa stack to 26.1.6.

Tested/not the cause: DXVK descriptor-heap disabled (dxvk.enableDescriptorHeap = False, log confirmed Binding model: Descriptor buffer) — artifact persisted; shader caches cleared (fossilize/vkd3d/radv disk cache) — no change.

### Steps to reproduce

Launch RE9 in gamescope session on VanGogh with mesa 26.2.1. Downgrade to 26.1.6 → resolved.

### Expected behavior

Game runs Normally

### Actual behavior

D3D12 games render with blocky/pixelated textures and text plus heavy specular shimmering in game mode

### Logs / stack traces

```text

```

### Additional system information

_No response_