vaapipostproc can not support RGBA to NV12 conversion

All RBGx kind of format can not be converted to other format in vaapipostproc.

gst-launch-1.0 videotestsrc ! capsfilter caps=video/x-raw,format=RGBA,framerate=30/1,width=640,height=360 ! vaapipostproc format=nv12 ! vaapisink