x86/agp: Fix amd64-agp module initialization regression

This fixes the regression introduced by commit
42590a75 ("x86/agp: Fix
agp_amd64_init and agp_amd64_cleanup").

The commit 61684cea fixed the
above regression but it's not enough. When amd64-agp is built as
a module, AGP isn't initialized, iommu is initialized, all the
aperture is owned by the iommu.

Reported-by: Marin Mitov <mitov@issp.bas.bg>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Tested-by: Marin Mitov <mitov@issp.bas.bg>
LKML-Reference: <20100204090802S.fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>