[media] vb2: fix vb2 state check when start_streaming fails

Commit bd994ddb (vb2: Fix stream start and
buffer completion race) broke the buffer state check in vb2_buffer_done.

So accept all three possible states there since I can no longer tell the
difference between vb2_buffer_done called from start_streaming or from
elsewhere.

Instead add a WARN_ON at the end of start_streaming that will check whether
any buffers were added to the done list, since that implies that the wrong
state was used as well.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: stable@vger.kernel.org      # for v3.15 and up
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>