intel_agp: fix GTT map size on G33

G33 has 1MB GTT table range.  Fix GTT mapping in case like 512MB aperture
size.

Signed-off-by: Zhenyu Wang <zhenyu.z.wang@intel.com>
Acked-by: Dave Airlie <airlied@linux.ie>
Cc: Dave Jones <davej@codemonkey.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>