media: drivers: use video_device_pipeline_alloc_start()

Use video_device_pipeline_alloc_start() instead of manually
allocating/managing the media pipeline storage.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>