V4L/DVB: videobuf: add ext_lock argument to the queue init functions

Add an ext_lock argument to the videobuf init functions. This allows
drivers to pass the vdev->lock pointer (or any other externally held lock)
to videobuf. For now all drivers just pass NULL.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>