[PATCH] x86-64: make GART PTEs uncacheable

This patches fixes the silent data corruption problems being seen using the
GART iommu where 4kB of data where incorrect (seen mostly on Nvidia CK804
systems).  This fix, to mark the memory regin the GART PTEs reside on as
uncacheable, also brings the code in line with the AGP specification.

Signed-off-by: Joachim Deguara <joachim.deguara@amd.com>
Signed-off-by: Andi Kleen <ak@suse.de>