iommu/ipmmu-vmsa: Rewrite page table management

The IOMMU core will only call us with page sizes advertized as supported
by the driver. We can thus simplify the code by removing loops over PGD
and PMD entries.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>