iommu/ipmmu-vmsa: Clean up bus_set_iommu()

Stop calling bus_set_iommu() since it's now unnecessary. This also
leaves the custom initcall effectively doing nothing but register
the driver, which no longer needs to happen early either, so convert
it to builtin_platform_driver().

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/r/14377566e449950c19367f75ec1b09724bf0889f.1660572783.git.robin.murphy@arm.com


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