iommu/amd: Clean up bus_set_iommu()

Stop calling bus_set_iommu() since it's now unnecessary, and
garbage-collect the last remnants of amd_iommu_init_api().

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/6bcc367e8802ae5a2b2840cbe4e9661ee024e80e.1660572783.git.robin.murphy@arm.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>