G33 has 1MB GTT table range.  Fix GTT mapping in case like 512MB aperture
size.
Signed-off-by: NZhenyu Wang <zhenyu.z.wang@intel.com>
Acked-by: NDave Airlie <airlied@linux.ie>
Cc: Dave Jones <davej@codemonkey.org.uk>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>