iommu/ipmmu-vmsa: PMD is never folded, PUD always is

The driver only supports the 3-level long descriptor format that has no
PUD and always has a PMD.

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