libs: display: egl: avoid two vaDisplay instantiates
libs: display: egl: avoid two vaDisplay instantiates

GstVaapiDisplayEGL is a wrapper of another GstVaapiDisplay, either X11
or Wayland. Nonetheless it created another vaDisplay for it, instead
of using the wrapped one.

This patch enables the reuse of the wrapped vaDisplay avoiding
instantiating two.

https://bugzilla.gnome.org/show_bug.cgi?id=782212

ceyusa/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,GStreamer/gstreamer-vaapi