iommu/amd: Init unity mappings only for dma_ops domains

The default domain for a device might also be
identity-mapped. In this case the kernel would crash when
unity mappings are defined for the device. Fix that by
making sure the domain is a dma_ops domain.

Fixes: 0bb6e243 ('iommu/amd: Support IOMMU_DOMAIN_DMA type allocation')
Cc: stable@vger.kernel.org # v4.2+
Signed-off-by: Joerg Roedel <jroedel@suse.de>