When the DEC_CMD_STOP command is issued after the context has already
consumed all the queued buffers, we need to make sure to wake the
destination queue with last_buffer_dequeued set, to allow userspace to
make progress in its EOS handling.

As there might still be picture run workers pending at that point, we
need to synchronize with them, so the sequence number comparison reads
stable values.

 reword commit message]
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>