iommu/amd: Free page-table in protection_domain_free()

Align release of the page-table with the place where it is allocated.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Reviewed-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Link: https://lore.kernel.org/r/20200527115313.7426-6-joro@8bytes.org