media: coda: fix last buffer handling in V4L2_ENC_CMD_STOP

coda_encoder_cmd() is racy, as the last scheduled picture run worker can
still be in-flight while the ENC_CMD_STOP command is issued. Depending
on the exact timing the sequence numbers might already be changed, but
the last buffer might not have been put on the destination queue yet.

In this case the current implementation would prematurely wake the
destination queue with last_buffer_dequeued=true, causing userspace to
call streamoff before the last buffer is handled.

Close this race window by synchronizing with the pic_run_worker before
doing the sequence check.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
[l.stach@pengutronix.de: switch to flush_work, reword commit message]
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>