dxva2_vc1: set refdist value according to spec

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/dxva2_vc1.c

@@ -122,7 +122,7 @@ static void fill_picture_parameters(AVCodecContext *avctx,
                                   (v->range_mapuv_flag << 3) |
                                   (v->range_mapuv          );
     pp->bPicBinPB               = 0;
-    pp->bMV_RPS                 = 0;
+    pp->bMV_RPS                 = (v->fcm == ILACE_FIELD && pp->bPicBackwardPrediction) ? v->refdist + 9 : 0;
     pp->bReservedBits           = 0;
     if (s->picture_structure == PICT_FRAME) {
         pp->wBitstreamFcodes        = v->lumscale;