drm: use list_for_each_entry in drm_mm.c

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Thomas Hellstrom <thellstrom@vmwgfx.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>