x86/agp: tighten check to update amd nb aperture

Impact: fix bug to make agp work with dri

Jeffrey reported that dri does work with 64bit, but doesn't work with
32bit it turns out NB aperture is 32M, aperture on agp is 128M

64bit is using 64M for vaidation for 64 iommu/gart 32bit is only using
32M..., and will not update the nb aperture.

So try to compare nb apterture and agp apterture before leaving not
touch nb aperture.

Reported-by: Jeffrey Trull <jetrull@sbcglobal.net>
Tested-by: Jeffrey Trull <jetrull@sbcglobal.net>
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Acked-by: Dave Airlie <airlied@linux.ie>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>