media: entity: Add lockdep check to media graph walk

It was always assumed that walking the media graph would require holding
the media_device graph_mutex but this was not documented nor checked for.

Add a lockdep check to graph walk init and iter, and document the need for
acquiring the graph_mutex.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>