[media] videobuf2-core: add helper functions

Add helper functions to make it easier to adapt drivers to vb2.

These helpers take care of core locking and check if the filehandle is the
owner of the queue.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>