iommu/amd: Move gart fallback to amd_iommu_init

The fallback to the GART driver in the case amd_iommu doesn't work was
executed in a function called free_iommu_resources, which didn't really
make sense. This was even being called twice if amd_iommu=off was
specified on the command line.

The only complication is that it needs to be verified that amd_iommu has
fully relinquished control by calling free_iommu_resources and emptying
the amd_iommu_list.

Signed-off-by: Kevin Mitchell <kevmitch@arista.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>