iommu/vt-d: Always reserve a domain ID for identity setup

We will use a global static identity domain. Reserve a static domain ID
for it.

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/20240809055431.36513-4-baolu.lu@linux.intel.com


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