x86, GART: Disable GART table walk probes

Current code tramples over bit F3x90[6] which can be used to
disable GART table walk probes. However, this bit should be set
for performance reasons (speed up GART table walks). We are
allowed to do that since we put GART tables in UC memory later
anyway. Make it so.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
LKML-Reference: <1283531981-7495-3-git-send-email-bp@amd64.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>