[AGPGART] VIA and SiS AGP chipsets are x86-only

There's no point in troubling the Alpha, IA-64, PowerPC and PARISC
people with SiS and VIA options.  Andrew thinks it helps find bugs,
but there's no evidence of that.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: Dave Jones <davej@redhat.com>