iommu: Drop sw_msi from iommu_domain

There are only two sw_msi implementations in the entire system, thus it's
not very necessary to have an sw_msi pointer.

Instead, check domain->cookie_type to call the two sw_msi implementations
directly from the core code.

Link: https://patch.msgid.link/r/7ded87c871afcbaac665b71354de0a335087bf0f.1742871535.git.nicolinc@nvidia.com


Suggested-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>