iommu: Clean up bus_set_iommu()

Clean up the remaining trivial bus_set_iommu() callsites along
with the implementation. Now drivers only have to know and care
about iommu_device instances, phew!

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Tested-by: Matthew Rosato <mjrosato@linux.ibm.com> # s390
Tested-by: Niklas Schnelle <schnelle@linux.ibm.com> # s390
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/ea383d5f4d74ffe200ab61248e5de6e95846180a.1660572783.git.robin.murphy@arm.com


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