agp/amd64: Remove GART dependency on AGP_AMD64

The GART IOMMU code has no strong dependency to the AMD64
AGP code. So the automatic selection of AGP_AMD64 for GART
can be removed.

Cc: Dave Jones <davej@redhat.com>
Signed-off-by: Pavel Vasilyev <pavel@pavlinux.ru>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>