[AGPGART] x86-64 Kconfig fixes.

[AGPGART] x86-64 Kconfig fixes.
Offering the K7 GARTs on 64bit kernels causes sillyness, like reports of
"unrecognised device, try unsupported".  We don't want people to even try
that, so don't offer it in the first place.

There's really no good reason for offering any of the IA32 era GARTs on a
x86-64 64bit kernel. If Intel (or whoever) ever do an x86-64 clone, a new
gart driver will be started anyways, as has been done for amd-k8-agp.