vaapipostproc: Add string representation of VPP functions to ElementFactoy Klass
vaapipostproc: Add string representation of VPP functions to ElementFactoy Klass

Added the same Klass specifications used in other upstream
video postprocessing elements like videoconvert, videoscale,
videobalance and deinterlace.
An example use case is for this is to help the playsink
to autoplug the hardware accelerated deinterlacer.

01org/iotg-lin-gfx-gstreamer-vaapi,GStreamer/gstreamer-vaapi,CapOM/gstreamer-vaapi,CapOM/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,CapOM/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,ceyusa/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi