drm/i915/gvt: Rework shadow page management code

drm/i915/gvt: Rework shadow page management code

This is a another big one and the GVT shadow page management code is
heavily refined.

The new code only use struct intel_vgpu_ppgtt_spt to represent a vgpu
shadow page table - w/ or wo/ a guest page associated with. A pure shadow
page (no guest page associated) will be used to shadow splited 2M huge
gtt. In this case, the spt.guest_page.gfn should be a zero.

To search a existed shadow page table, we have two new interfaces:
 - intel_vgpu_find_spt_by_gfn(), find a spt by guest gfn. It must not
   be a pure spt.
 - intel_vgpu_find_spt_by_mfn, Find the spt using shadow page mfn in
   shadowed PTE.

The oos_page management is remained as what is was.

v2: Split some changes into small standalone patches.

Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>