Fairly commonly we want to inspect the node list on the struct drm_mm,
which is buried within an embedded node. Bring it to the surface with a
bit of syntatic sugar.

Note this was intended to be split from commit ad579002 ("drm: Add
drm_mm_for_each_node_safe()") before being applied, but my timing sucks.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161222083641.2691-3-chris@chris-wilson.co.uk