Fix the length value of VEB_DI_IECP.

Fixes: be1da707 ("drm/i915/gvt: vGPU command scanner")
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Gao, Fred <fred.gao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>