GPU usage spikes to 100 when decoding video in Wayland (not in X11)

When HW-accelerated decoding video is performed using VA-API on a Radeon 680M in Wayland, GPU usage keeps spiking to 100 % increasing power draw etc.

These spikes only occur during HW-accelerated decoding and are independent of available GPU resources (DPM set to low/GPU frequency capped at 200 MHz behaves the same as no cap at 2200 MHz - only power consumption increases accordingly as the spikes can now use more GPU), DE/WM used, codec, resolution, energy performance policy, AMD p-state settings, adaptive sync, refresh rate etc. As soon as VA-API playback ends, the spikes also disappear.

Example compared to X11 when playing a VP9 video:
- Chromium-based browsers, vlc and most other apps have avg GPU usage comparable to X11 but there are short spikes to 100 increasing total power consumption by 2 W (at low DPM) or up to 5 W when uncapped.
- Firefox-based browsers spike all the time increasing the avg GPU load six-fold with a 5 W consumption increase even at 200 MHz.

Reported HW decoder usage percentage is roughly the same in X11 and Wayland.

```
vainfo: VA-API version: 1.21 (libva 2.22.0)
vainfo: Driver version: Mesa Gallium driver 24.1.4-arch1.2 for AMD Radeon 680M (radeonsi, rembrandt, LLVM 18.1.8, DRM 3.57, 6.10.1-arch1-1)
vainfo: Supported profile and entrypoints
VAProfileH264ConstrainedBaseline: VAEntrypointVLD
VAProfileH264ConstrainedBaseline: VAEntrypointEncSlice
VAProfileH264Main : VAEntrypointVLD
VAProfileH264Main : VAEntrypointEncSlice
VAProfileH264High : VAEntrypointVLD
VAProfileH264High : VAEntrypointEncSlice
VAProfileHEVCMain : VAEntrypointVLD
VAProfileHEVCMain : VAEntrypointEncSlice
VAProfileHEVCMain10 : VAEntrypointVLD
VAProfileHEVCMain10 : VAEntrypointEncSlice
VAProfileJPEGBaseline : VAEntrypointVLD
VAProfileVP9Profile0 : VAEntrypointVLD
VAProfileVP9Profile2 : VAEntrypointVLD
VAProfileAV1Profile0 : VAEntrypointVLD
VAProfileNone : VAEntrypointVideoProc
```
The issue also occurred on kernel 6.9 and with earlier mesa drivers.

Thank you very much for your help!