agp/intel: fix physical address mask bits for sandybridge

It should shift bit 39-32 into pte's bit 11-4.

Reported-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: stable@kernel.org
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>