[media] mtk-vcodec: add index check in decoder vidioc_qbuf

vb2_qbuf will check the buffer index. If a driver overrides
vidioc_qbuf and use the buffer index, the driver needs to check
the index.

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