intel-iommu: Fix reference by physical address in intel_iommu_attach_device()

Commit a99c47a2 "intel-iommu: errors with smaller iommu widths" replace the
dmar_domain->pgd with the first entry of page table when iommu's supported
width is smaller than dmar_domain's. But it use physical address directly
for new dmar_domain->pgd...

This result in KVM oops with VT-d on some machines.

Reported-by: Allen Kay <allen.m.kay@intel.com>
Cc: Tom Lyon <pugs@cisco.com>
Signed-off-by: Sheng Yang <sheng@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>