iommu/amd: Optimize iommu_unmap_page for new fetch_pte interface

Now that fetch_pte returns the page-size of the pte, this
function can be optimized a lot.

Tested-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>