dnxhd codec is intra only

Originally committed as revision 15672 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/utils.c

@@ -746,6 +746,7 @@ static int is_intra_only(AVCodecContext *enc){
         case CODEC_ID_ASV1:
         case CODEC_ID_ASV2:
         case CODEC_ID_VCR1:
+        case CODEC_ID_DNXHD:
             return 1;
         default: break;
         }