display-gst: check GstRegistry for decoding elements

With this patch, we can find all the elements in the registry that are
video decoders which can handle the predefined GstCaps.

Main benefits are:
- We don't rely on predefined list of GstElements. We don't need to
  update them;
- debugging: It does help to know what the registry has at runtime;

Signed-off-by: Victor Toso <victortoso@redhat.com>
Acked-by: Frediano Ziglio <fziglio@redhat.com>