AGP: Warn when GATT memory cannot be set to UC

This is one of those paranoid checks which should at least tell
us that something is about to go haywire after we've disabled
GART table walk probes which is done by default now on AMD.

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: <1283531981-7495-4-git-send-email-bp@amd64.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>