drm/i915: replace ALIGN(PAGE_SIZE) by PAGE_ALIGN

use mm.h definition

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>