omxvideoenc: factor out gst_omx_video_enc_set_to_idle()
omxvideoenc: factor out gst_omx_video_enc_set_to_idle()

No semantic change. We'll have to use this when the input pool is
activated so we can allocate buffers.

https://bugzilla.gnome.org/show_bug.cgi?id=796918

GStreamer/gst-omx,GStreamer/gst-omx,GStreamer/gst-omx