iommu/vt-d: Avoid sending invalid page response

Page responses should only be sent when last page in group (LPIG) or
private data is present in the page request. This patch avoids sending
invalid descriptors.

Fixes: 5d308fc1 ("iommu/vt-d: Add 256-bit invalidation descriptor support")
Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>