iommu/amd: Add helper functions to update domain->pt_root

Do not call atomic64_set() directly to update the domain page-table
root and use two new helper functions.

This makes it easier to implement additional work necessary when
the page-table is updated.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Link: https://lore.kernel.org/r/20200626080547.24865-2-joro@8bytes.org