[bisected] artifacts in vaapi h.264 playback

RX 6900XT.

be28a475 is the first bad commit

To reproduce with for example firefox (96 here):

1. Enable media.ffmpeg.vaapi.enabled in about:config and restart firefox.
2. Drop a h.264 video on firefox.

Example video:
Peek_2021-12-26_23-39

The first few frames should show heavy artifacts, before it clears up and plays the rest of the video fine.