fix typo

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@2834 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavcodec/xvmcvideo.c

@@ -298,7 +298,7 @@ const int mb_xy = s->mb_y * s->mb_stride + s->mb_x;
 
     blocks_per_mb = 6;
     if( s->chroma_format >= 2){
-        block_per_mb = 4 + (1 << (s->chroma_format));
+        blocks_per_mb = 4 + (1 << (s->chroma_format));
     }
 
     if(s->flags & CODEC_FLAG_GRAY){