media: vb2: use lock if wait_prepare/finish are NULL

If the wait_prepare or wait_finish callback is set, then call it.
If it is NULL and the queue lock pointer is not NULL, then just
unlock/lock that mutex.

This allows simplifying drivers by dropping the wait_prepare and
wait_finish ops (and eventually the vb2_ops_wait_prepare/finish helpers).

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>