media: mc: entity: Add media_entity_pipeline() to access the media pipeline

Replace direct access to the pipe field in drivers with a new helper
function. This will allow easier refactoring of media pipeline handling
in the MC core behind the scenes without affecting drivers.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
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>