Revert "agp: AMD AGP is used on UP1100 & UP1500 alpha boxen"

This reverts commit f191f144.

The AMD 751 and 761 chipsets are used on the UP1000, UP1100, and UP1500
OEM motherboards, but they neglect to do anything to make AGP work.

According to Ivan Kokshaysky:
There is quite fundamental conflict between the Alpha architecture and
x86 AGP implementation - Alpha is entirely cache coherent by design,
while x86 AGP is not (I mean native AGP DMA transactions, not a PCI over
AGP). There are no such things as non-cacheable mappings or software
support for cache flushing/invalidation on Alpha, so x86 AGP code won't
work on Nautilus.

So there's no point in allowing this driver to be configured on Alpha.

Signed-off-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>