drm/i915/gvt: add gtt_invalidate API to flush the GTT TLB

add gtt_invalidate API to handle the GTT TLB flush instead of
hiding in write_pte64 function. This can avoid overkill when using
write_pte64

Suggested-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>