media: mediatek: vdec: Remove useless loop

Simplify code by removing useless loop by using video buffer index.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Reviewed-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
CC: Bin Liu <bin.liu@mediatek.com>
CC: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>