media: coda: flush bitstream ring buffer on decoder restart

The bitstream ringbuffer might be in an underrun state after draining,
or it might still contain unread data if the previous decoder stop
command was flagged as immediate. Flush the bitstream ring buffer
during V4L2_DEC_CMD_START to get into a well defined state. Also fill
the bitstream with buffers that have been queued during draining,
to resume decoding immediately.

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>