media: staging: media: imx: imx7-media-csi: Inline pipeline start/stop

The imx7_csi_media_pipeline_set_stream() function has separate branches
for the start and stop paths, surrounded by mutex lock/unlock. That is
very little shared code, inline the locking and corresponding branch in
each of the two callers.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>