drm: Remove mtrr_add and mtrr_del fallback hack for non-MTRR systems

There are no users left in drivers/gpu.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>