intel-agp: Switch to wbinvd_on_all_cpus

Simplify if-statement while at it.

[ hpa: we need to #include <asm/smp.h> ]

Cc: Dave Jones <davej@redhat.com>
Cc: David Airlie <airlied@linux.ie>
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>
LKML-Reference: <1264172467-25155-3-git-send-email-bp@amd64.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>