x86/gart: Do not select AGP for GART_IOMMU

There is no dependency from the gart code to the agp code.
And since a lot of systems today do not have agp anymore
remove this dependency from the kernel configuration.

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