drm/i915/gvt: introuduce intel_vgpu_reset_gtt() to reset gtt

This patch introduces a new function intel_vgpu_reset_gtt() to reset
the all GTT related status, including GGTT, PPGTT, scratch page. This
function can free all shadowed PPGTT, clear all GGTT entry, and clear
scratch page to all zero. After this, we can ensure no gtt related
information can be leakaged from one VM to anothor one when assign
vgpu instance across different VMs (not simultaneously).

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