iommu/amd: Allow downgrading page-sizes in alloc_pte()

Before this patch it was not possible the downgrade a
mapping established with page-mode 7 to a mapping using
smaller page-sizes, because the pte_level != level check
prevented that.

Treat page-mode 7 like a non-present mapping and allow to
overwrite it in alloc_pte().

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