[media] vb2: allow requeuing buffers while streaming

vb2_buffer_done() already allows STATE_QUEUED, but currently only when not
streaming. It is useful to allow it while streaming as well, as this makes
it possible for drivers to requeue buffers while waiting for a stable
video signal.

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