vtdec: Negotiate the decoder in set_format() already

Don't wait until later, we want to know here if the codec can be opened or not
for the requested format. This was removed (accidentially?) by
  119e09eac315f79ac2cf45b4441ad1d932130614

Without this decodebin has no way to switch to a different decoder if this one
does not work.

https://bugzilla.gnome.org/show_bug.cgi?id=762613

sys/applemedia/vtdec.c

@@ -359,7 +359,7 @@ gst_vtdec_set_format (GstVideoDecoder * decoder, GstVideoCodecState * state)
     gst_video_codec_state_unref (vtdec->input_state);
   vtdec->input_state = gst_video_codec_state_ref (state);
 
-  return TRUE;
+  return gst_video_decoder_negotiate (decoder);
 }
 
 static gboolean