iommu/arm-smmu: Treat IOMMU_DOMAIN_DMA as bypass for now

Until all upstream devices have their DMA ops swizzled to point at the
SMMU, we need to treat the IOMMU_DOMAIN_DMA domain as bypass to avoid
putting devices into an empty address space when detaching from VFIO.

Signed-off-by: Will Deacon <will.deacon@arm.com>