iommu/vt-d, trivial: print correct domain id of static identity domain

Field si_domain->id is set by iommu_attach_domain(), so we should only
print domain id for static identity domain after calling
iommu_attach_domain(si_domain, iommu), otherwise it's always zero.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>