[media] media: Enforce single entity->pipe in a pipeline

If a different entity->pipe in a pipeline was encountered, a warning was
issued but the execution continued as if nothing had happened. Return an
error instead right there.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>