drm/i915/gvt: Map shadow page before using it in shadow page table

MFN usually refers to "Machine Frame Number" in virtulization world.
Currently GVT-g populates the shadow PPGTT/GGTT page table with MFN
according to the translation between MFN and Guest PFN.

When host IOMMU is enabled, GPU DMA transactions go through the IOMMU,
GPU needs an IOVA<->MFN mapping to walk the shadow page table in host
main memory.

This patch will map a page in IOMMU page table before using it in shadow
page table and release the map when a shadow page is going to be freed.

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