iommu/mediatek: Use accessor functions for iommu private data

Make use of dev_iommu_priv_set/get() functions.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Reviewed-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Link: https://lore.kernel.org/r/20200326150841.10083-14-joro@8bytes.org