media: coda: implement CMD_START to restart decoding

The CMD_START shall be used to start the processing after a drain that
was initiated with CMD_STOP.

Up until now, a drain on coda could only be finished with a
STREAMOFF-STREAMON, which resulted in a reset of the device.

Signed-off-by: Michael Tretter <m.tretter@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>