[AGPGART] Suspend/Resume support for AMD64 GART.

This adds support for suspend/resume to the amd64-agp driver. Without
it, X displays garbage after resume from swsusp.

Signed-off-by: Michal Schmidt <xschmi00@stud.feec.vutbr.cz>
Cc: Andi Kleen <ak@muc.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Dave Jones <davej@redhat.com>