libs: encoder: correct encoder's ref/unref function.
libs: encoder: correct encoder's ref/unref function.

GstVaapiEncoder now is a standard gstobject and need to use
gst_object_ref/unref functions.

ceyusa/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,GStreamer/gstreamer-vaapi