ffmpeg: make elements reusable after registry rescan

ffmpeg: make elements reusable after registry rescan

If the same instance of the plugin is asked to be initialised more that once,
instances after the first one do not register the elements properly and the
elements become not usable.

For example, if you call gst_update_registry (), is not possible to create
elements after the call since the plugin is asked to be initialised again and
does not register the elements.

Fixes #584291