x86/agp: Fix agp_amd64_init regression

This fixes the regression introduced by commit
42590a75

 ("x86/agp: Fix
agp_amd64_init and agp_amd64_cleanup").

The above commit changes agp_amd64_init() not to do anything if
gart_iommu_aperture is not zero.

If GART iommu calls agp_amd64_init(), we need to skip
agp_amd64_init() when it's called later via module_init.

The problem is that gart_iommu_init() calls agp_amd64_init()
with not zero gart_iommu_aperture so agp_amd64_init() is never
initialized.

When gart_iommu_init() calls agp_amd64_init(), agp should be
always initialized.

Reported-by: Marin Mitov <mitov@issp.bas.bg>
Reported-by: Johannes Hirte <johannes.hirte@fem.tu-ilmenau.de>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Tested-by: Marin Mitov <mitov@issp.bas.bg>
Tested-by: Kevin Winchester <kjwinchester@gmail.com>
Cc: davej@redhat.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
LKML-Reference: <20100125141006O.fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>