x86/amd-iommu: Use only per-device dma_ops

Unfortunatly there are systems where the AMD IOMMU does not
cover all devices. This breaks with the current driver as it
initializes the global dma_ops variable. This patch limits
the AMD IOMMU to the devices listed in the IVRS table fixing
DMA for devices not covered by the IOMMU.

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