x86, GART: Remove superfluous AMD64_GARTEN

There is a GARTEN so use that and drop the duplicate.

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: <1283531981-7495-2-git-send-email-bp@amd64.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>