Use the parent display object, no need to maintain another one.
Use the parent display object, no need to maintain another one.

In the end, libgstvaapi-x11 reduced by 1 KB in .text vs. 0.1.1.


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