drm/i915/gvt: Factor out intel_vgpu_{get, put}_ppgtt_mm interface

Factor out these two interfaces so we can kill some duplicated code in
scheduler.c.

v2:
  - rename to intel_vgpu_{get,put}_ppgtt_mm
  - refine handle_g2v_notification

Signed-off-by: Changbin Du <changbin.du@intel.com>
Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>