iommu/amd: Collect page-table pages in freelist

Collect all pages that belong to a page-table in a list and
free them after the tree has been traversed. This allows to
implement safer page-table updates in subsequent patches.
Also move the functions for page-table freeing a bit upwards
in the file so that they are usable from the iommu_map() path.

Signed-off-by: Joerg Roedel <jroedel@suse.de>