Current size of prev_refs is 8, which just means the size of ref-frames
but needs to be aligned with full size of dpb, which is 9.
Also prev_refs is now indexed by dpb slot and holds the last intra frame
written to that slot.
This fixes visible artifacts on AV1 streams that mix super-res and
non-super-res frames in a hierarchical reference structure.
Closes: mesa/mesa#15503
Signed-off-by: Hyunjun Ko <zzoon@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/41846>