iommu/amd: Restructure code for freeing page table

By consolidate logic into v1_free_pgtable helper function,
which is called from IO page table framework.

Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Link: https://lore.kernel.org/r/20201215073705.123786-8-suravee.suthikulpanit@amd.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>