Since there's a v4l2_m2m_get_vq helper, use it instead of accessing the
queue directly, which slightly increases readability.

Also reformat the code using the queue a bit while at it.
Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>