[media] uvcvideo: Allow userptr IO mode

Userptr can be very useful if a UVC camera is requested to use video
buffers allocated by another processing device. So that buffers don't
need to be copied.

Signed-off-by: Javier Martin <javier.martin@vista-silicon.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>