drm: Add drm_mm_for_each_node_safe()

A complement to drm_mm_for_each_node(), wraps list_for_each_entry_safe()
for walking the list of nodes safe against removal.

Note from Joonas:

"Most of the diff is about __drm_mm_nodes(mm), which could be split into
own patch and keep the R-b's."

But I don't feel like insisting on the resend.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
[danvet: Add note.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161216074718.32500-4-chris@chris-wilson.co.uk