drm/i915: Enable platform support for vGPU huge gtt pages

Now GVTg supports shadowing both 2M/64K huge gtt pages. So let's turn on
the cap info bit VGT_CAPS_HUGE_GTT.

v2: Split changes in i915 side into a separated patch.

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