iommu/amd: Allow to upgrade page-size

Before this patch the iommu_map_page() function failed when
it tried to map a huge-page where smaller mappings existed
before.

With this change the page-table pages of the old mappings
are teared down, so that the huge-page can be mapped.

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