media: v4l2-ioctl.c: Simplify locking for m2m devices

Now that the mutexes for output and capture vb2 queues match,
it is possible to refer to the context q_lock as the
m2m lock for a given m2m context.

Remove the output/capture lock selection.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>