iommu/amd: Clean up function declarations

Remove unused declarations and add static keyword as needed.

Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Vasant Hegde <vasant.hegde@amd.com>
Link: https://lore.kernel.org/r/20220301085626.87680-4-vasant.hegde@amd.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>