[media] media: entity: Split graph walk iteration into two functions

With media_entity_graph_walk_next() getting more and more complicated (and
especially so with has_routing() support added), split the function into
two.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>