drm: Add drm_mm_for_each_node_safe()

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 <[email protected]>
Reviewed-by: Joonas Lahtinen <[email protected]>
[danvet: Add note.]
Signed-off-by: Daniel Vetter <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]