media: dvb-core: dvb_vb2: drop wait_prepare/finish callbacks

media: dvb-core: dvb_vb2: drop wait_prepare/finish callbacks

Since commit 88785982a19d ("media: vb2: use lock if wait_prepare/finish
are NULL") it is no longer needed to set the wait_prepare/finish
vb2_ops callbacks as long as the lock field in vb2_queue is set.

Set the queue lock to &ctx->mutex, which makes it possible to drop
the wait_prepare/finish callbacks.

This simplifies the code and this is a step towards the goal of deleting
these callbacks.

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