iommu: Refactor __iommu_domain_alloc()

Following patch will introduce iommu_paging_domain_alloc_flags() API.
Hence move domain init code to separate function so that it can be
reused.

Also move iommu_get_dma_cookie() setup iommu_setup_default_domain() as
it is required in DMA API mode only.

Signed-off-by: Jason Gunthorpe <jgg@ziepe.ca>
[Split the patch and added description - Vasant]
Signed-off-by: Vasant Hegde <vasant.hegde@amd.com>
Reviewed-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: Yi Liu <yi.l.liu@intel.com>
Link: https://lore.kernel.org/r/20241028093810.5901-2-vasant.hegde@amd.com


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