[media] v4l2-mem2mem: drop rdy_queue on STREAMOFF

When a v4l2-mem2mem context gets a STREAMOFF call on either its CAPTURE
or OUTPUT queues, we should:
* Drop the corresponding rdy_queue, since a subsequent STREAMON expects
  an empty queue.
* Deschedule the context, as it now has at least one empty queue and
  cannot run.

Signed-off-by: John Sheu <sheu@google.com>
Acked-by: Pawel Osciak <pawel@osciak.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>