iommu/amd: Optimize alloc_new_range for new fetch_pte interface

Now that fetch_pte returns the page-size of the pte, the
call in this function can also be optimized a little bit.

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