x86/amd-iommu: Fall back to GART if initialization fails

This patch implements a fallback to the GART IOMMU if this
is possible and the AMD IOMMU initialization failed.
Otherwise the fallback would be nommu which is very
problematic on machines with more than 4GB of memory or
swiotlb which hurts io-performance.

Cc: stable@kernel.org
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>