Our VM code already has a cleanup function, and this is a nice place to
put the drm_mm_takedown. This should have no functional impact, it just
leaves the unload function a bit cleaer, and is more logical IMO
Signed-off-by: NBen Widawsky <ben@bwidawsk.net>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>