iommu/vt-d: Simplify check in identity_mapping()

The function only has one call-site and there it is never called with
dummy or deferred devices. Simplify the check in the function to
account for that.

Fixes: 1ee0186b ("iommu/vt-d: Refactor find_domain() helper")
Cc: stable@vger.kernel.org # v5.5
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Acked-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>