[media] media: Always keep a graph walk large enough around

Re-create the graph walk object as needed in order to have one large enough
available for all entities in the graph.

This enumeration is used for pipeline power management in the future.

[mchehab@osg.samsung.com: fix documentation bug:
 " warning: bad line: graph_mutex"]

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