media: videobuf2-core: update vb2_thread if wait_finish/prepare are NULL

The vb2_thread is used for DVB support. This will queue and dequeue buffers
automatically.

It calls wait_finish/prepare around vb2_core_dqbuf() and vb2_core_qbuf(),
but that assumes all drivers have these ops set. But that will change
due to commit 88785982 ("media: vb2: use lock if wait_prepare/finish
are NULL").

So instead just check if the callback is available, and if not, use
q->lock, just as __vb2_wait_for_done_vb() does.

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