Can not covert RGBx and RGBA to NV12 using vaapipostproc for iHD driver

gst-launch-1.0 videotestsrc ! capsfilter caps=video/x-raw,format=RGBx,width=800,height=600 ! vaapipostproc ! video/x-raw,format=NV12 ! vaapisink

failed on iHD driver But i965 can work