iommu/amd: Allocate page-table in protection_domain_init()

Consolidate the allocation of the domain page-table in one place.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Reviewed-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Link: https://lore.kernel.org/r/20200527115313.7426-5-joro@8bytes.org