media: mtk-vcodec: vdec: clean up vidioc_vdec_s_fmt a bit

Check for a potentially NULL pointer that was overlooked and use shorter
accessors to the same data. While we are at it, sprinkle a few comments.

Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>