vaapipostproc: don't require a vaapi display for all caps queries
vaapipostproc: don't require a vaapi display for all caps queries

This delays the requirement of having a GstVaapiDisplay until later

https://bugzilla.gnome.org/show_bug.cgi?id=768161

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