omxvideoenc: factor out gst_omx_video_enc_nv12_manual_copy()
omxvideoenc: factor out gst_omx_video_enc_nv12_manual_copy()

No semantic change, I'm going to re-use it to copy the NV12_10LE32
format.

https://bugzilla.gnome.org/show_bug.cgi?id=793694

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