Use single reference frame for 4x4 coding block in chroma-sub8x8

Change-Id: Ic04bd1ac62ea35082883d0560f91843040e35a64
diff --git a/av1/common/blockd.h b/av1/common/blockd.h
index 9dde704..5f89bc3 100644
--- a/av1/common/blockd.h
+++ b/av1/common/blockd.h
@@ -38,7 +38,11 @@
 extern "C" {
 #endif
 
-#define SUB8X8_COMP_REF (!(CONFIG_CB4X4 && CONFIG_CHROMA_2X2))
+#if (CONFIG_CHROMA_SUB8X8 || CONFIG_CHROMA_2X2)
+#define SUB8X8_COMP_REF 0
+#else
+#define SUB8X8_COMP_REF 1
+#endif
 
 #define MAX_MB_PLANE 3