[media] mtk-vcodec: check the vp9 decoder buffer index from VPU

VPU firmware has a bug and may return invalid buffer index for
some vp9 videos. Check the buffer indexes before accessing the
buffer.

Signed-off-by: Wu-Cheng Li <wuchengli@chromium.org>
Acked-by: Tiffany Lin <Tiffany.lin@mediatek.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>