display: use prefixed display names for cache lookups.
display: use prefixed display names for cache lookups.

This improves display name comparisons by always allocating a valid display
name. This also helps to disambiguate lookups by name in the global display
cache, should a new backend be implemented.

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