dnxhddec: indicate colorspace

It is supposed to only old BT.709 colorspaces.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/dnxhddec.c

@@ -73,6 +73,7 @@ static av_cold int dnxhd_decode_init(AVCodecContext *avctx)
 
     ctx->avctx = avctx;
     ctx->cid = -1;
+    avctx->colorspace = AVCOL_SPC_BT709;
     return 0;
 }