Reduce the verbosity of a mpeg1/2 invalid intra-matrix warning.

Fixes ticket #973.

libavcodec/mpeg12.c

@@ -1497,7 +1497,7 @@ static int load_matrix(MpegEncContext *s, uint16_t matrix0[64], uint16_t matrix1
             return -1;
         }
         if (intra && i == 0 && v != 8) {
-            av_log(s->avctx, AV_LOG_ERROR, "intra matrix specifies invalid DC quantizer %d, ignoring\n", v);
+            av_log(s->avctx, AV_LOG_DEBUG, "intra matrix specifies invalid DC quantizer %d, ignoring\n", v);
             v = 8; // needed by pink.mpg / issue1046
         }
         matrix0[j] = v;