iommu/arm-smmu: Use accessor functions for iommu private data

Make use of dev_iommu_priv_set/get() functions and simplify the code
where possible with this change.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Tested-by: Will Deacon <will@kernel.org> # arm-smmu
Link: https://lore.kernel.org/r/20200326150841.10083-12-joro@8bytes.org