iommu: Split iommu_group_remove_device() into helpers

So that code could be re-used by iommu_release_device() in the subsequent
change. No intention for functionality change.

Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20230322064956.263419-3-baolu.lu@linux.intel.com


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