iommu/vt-d: Split intel_iommu_domain_alloc_paging_flags()

iommu/vt-d: Split intel_iommu_domain_alloc_paging_flags()

Create stage specific functions that check the stage specific conditions
if each stage can be supported.

Have intel_iommu_domain_alloc_paging_flags() call both stages in sequence
until one does not return EOPNOTSUPP and prefer to use the first stage if
available and suitable for the requested flags.

Move second stage only operations like nested_parent and dirty_tracking
into the second stage function for clarity.

Move initialization of the iommu_domain members into paging_domain_alloc().

Drop initialization of domain->owner as the callers all do it.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/4-v3-dbbe6f7e7ae3+124ffe-vtd_prep_jgg@nvidia.com
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/20250714045028.958850-7-baolu.lu@linux.intel.com
Signed-off-by: Will Deacon <will@kernel.org>