iommu/vt-d: Factor out helpers from domain_context_mapping_one()

Extract common code from domain_context_mapping_one() into new helpers,
making it reusable by other functions such as the upcoming identity domain
implementation. No intentional functional changes.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Link: https://lore.kernel.org/r/20240809055431.36513-6-baolu.lu@linux.intel.com


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