obs-ffmpeg: Check correct VA-API codec support

Previously, we just checked for H264 regardless of which codec was
selected. This mostly worked, but on Fedora they ship AV1 without H264
or HEVC. In that case users wont see AV1 as an option.
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f