x86/agp: Fix agp_amd64_init and agp_amd64_cleanup

This fixes the regression introduced by the commit
f405d2c02395a74d3883bd03ded36457aa3697ad.

The above commit fixes the following issue:

  http://marc.info/?l=linux-kernel&m=126192729110083&w=2

However, it doesn't work properly when you remove and insert the
agp_amd64 module again.

agp_amd64_init() and agp_amd64_cleanup should be called only
when gart_iommu is not called earlier (that is, the GART IOMMU
is not enabled). We need to use 'gart_iommu_aperture' to see if
GART IOMMU is enabled or not.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: mitov@issp.bas.bg
Cc: davej@redhat.com
LKML-Reference: <20100104161603L.fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>