iommu: Cleanup iommu_change_dev_def_domain()

As the singleton group limitation has been removed, cleanup the code
in iommu_change_dev_def_domain() accordingly.

Documentation is also updated.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20230322064956.263419-7-baolu.lu@linux.intel.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>