gstreamer: Added option to disable plugin registry.

The plugin registry can take a while to be generated when GStreamer is
initialized. Turning it off can speed up up GStreamer application launch
times. Default behaviour is unchanged.

Signed-off-by: Tim Sheridan <tim.sheridan@imgtec.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>