iommu/amd: Let free_pagetable() not rely on domain->pt_root

Use 'struct domain_pgtable' instead to free_pagetable(). This solves
the problem that amd_iommu_domain_direct_map() needs to restore
domain->pt_root after the device table has been updated just to make
free_pagetable release the domain page-table.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Reviewed-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Link: https://lore.kernel.org/r/20200527115313.7426-4-joro@8bytes.org