iommu/amd: Remove redundant NULL check before dma_ops_domain_free().

dma_ops_domain_free on a NULL pointer is a no-op, so the NULL check in
amd_iommu_init_dma_ops() can be removed.

Signed-off-by: Cyril Roelandt <tipecaml@gmail.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>