[media] v4l: vb: remove warnings about MEMORY_DMABUF

Adding DMABUF memory type causes videobuf to complain about not using it
in some switch cases. This patch removes these warnings.

Signed-off-by: Sumit Semwal <sumit.semwal@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Tested-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>