drm/mm: rewrite drm_mm_for_each_hole

When backwards is 0, __drm_mm_for_each_hole is same as
drm_mm_for_each_hole. So I rewrite drm_mm_for_each_hole
by using __drm_mm_for_each_hole.

Signed-off-by: Geliang Tang <geliangtang@163.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>