multimedia/libva: backport some upstream fixes
- https://github.com/intel/libva/commit/04acc7860feb will be required by libva-intel-media-driver-20.2.0, see https://github.com/intel/media-driver/commit/2baead9b10cd
- https://github.com/intel/libva/commit/48f446b32a4f is a fix for regression in libva-2.6.0