vp9_decodeframe: simplify compare_tile_buffers

return the difference between the 2 buffer sizes rather than exactly
-1/0/1.

Change-Id: Idf1ccff7088b31845470bcc71bea5927b0598cc7

vp9/decoder/vp9_decodeframe.c

@@ -1086,13 +1086,7 @@ static int tile_worker_hook(TileWorkerData *const tile_data,
 static int compare_tile_buffers(const void *a, const void *b) {
   const TileBuffer *const buf1 = (const TileBuffer*)a;
   const TileBuffer *const buf2 = (const TileBuffer*)b;
-  if (buf1->size < buf2->size) {
-    return 1;
-  } else if (buf1->size == buf2->size) {
-    return 0;
-  } else {
-    return -1;
-  }
+  return (int)(buf2->size - buf1->size);
 }
 
 static const uint8_t *decode_tiles_mt(VP9Decoder *pbi,