vaapisink: Fix rendering in drm display
vaapisink: Fix rendering in drm display

Make sure vaapisink create a va surface backed buffer pool and all
required attributes get assigned correctly for drm display type.

This is needed to make the below pipeline working:
gst-launch-1.0 filesrc location= raw_video.mov ! videoparse format=uyvy
width=320 height=240 framerate=30/1 ! vaapisink display=drm

https://bugzilla.gnome.org/show_bug.cgi?id=786954

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