drm/i915/gvt: map pfn for PTE entry in kvm

When host i915 iommu enabled, gvt needs to use a mapped pfn in PTE entry
So before kvm returns the pfn, map this pfn and return the mapped address
which is so called iova.

Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>