use mm.h definition

Cc: David Airlie <airlied@linux.ie>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>