autodetect: bring the element state down after success
autodetect: bring the element state down after success

Otherwise some messages that are emitted by the element on NULL->READY
will not reach the application.

https://bugzilla.gnome.org/show_bug.cgi?id=764947

hizukiayaka/gst-plugins-good,stfl/gst-plugins-good,GrokImageCompression/gst-plugins-good,hizukiayaka/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,hizukiayaka/gst-plugins-good,GStreamer/gst-plugins-good,hizukiayaka/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good