iommu/amd: Merge private header files

Merge amd_iommu_proto.h into amd_iommu.h.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Reviewed-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Link: https://lore.kernel.org/r/20200527115313.7426-9-joro@8bytes.org