drm/i915/gvt: Add drm_format_mod update

Add drm_format_mod update, which is omitted.

Fixes: e546e281

("drm/i915/gvt: Dmabuf support for GVT-g")
Cc: stable@vger.kernel.org
Signed-off-by: Tina Zhang <tina.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>