iommu/amd: Re-define amd_iommu_domain_encode_pgtable as inline

Move the function to header file to allow inclusion in other files.

Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Link: https://lore.kernel.org/r/20201215073705.123786-2-suravee.suthikulpanit@amd.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>