plugins: fix gstgl and vaapi memory leaks
plugins: fix gstgl and vaapi memory leaks

1\ Unref gl_display and gl_window as soon they are not needed.
2\ Remove an unneeded display type check, since is handled by
   gst_vaapi_created_display_from_handle()
3\ Unref vaapi's display if the display cannot be bind to a GL API.

Modified-by: Víctor Manuel Jáquez Leal <victorx.jaquez@intel.com>

https://bugzilla.gnome.org/show_bug.cgi?id=763354

ceyusa/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,GStreamer/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi