Avoid using MRC_DCT when the mask produced is invalid

If the mask is invalid, do not allow the encoder to select MRC_DCT.
Currently the mask is invalid if it is all 1 or all 0, but these
criteria will likely expand in a future patch.

Change-Id: I77230ea8357bfdb2bf1e6338903d44bbf1db22d1
diff --git a/av1/common/blockd.h b/av1/common/blockd.h
index 2e2474b..7fc7b77 100644
--- a/av1/common/blockd.h
+++ b/av1/common/blockd.h
@@ -377,6 +377,10 @@
 #endif                      // CONFIG_SUPERTX
   int8_t seg_id_predicted;  // valid only when temporal_update is enabled
 
+#if CONFIG_MRC_TX
+  int valid_mrc_mask;
+#endif  // CONFIG_MRC_TX
+
   // Only for INTRA blocks
   UV_PREDICTION_MODE uv_mode;
 #if CONFIG_PALETTE