Allow global motion based translation for sub8x8

Also handle block centers for sub8x8 blocks correctly.

BDRATE: lowres: -1.308%

Change-Id: I6bdb25bdaceb5086a8b951617fd62ed41cd89023
diff --git a/av1/common/blockd.h b/av1/common/blockd.h
index 6d3bfbb..9d044fe 100644
--- a/av1/common/blockd.h
+++ b/av1/common/blockd.h
@@ -1131,6 +1131,9 @@
     if (mbmi->mode != ZEROMV) return 0;
 #endif  // CONFIG_EXT_INTER
   } else {
+#if !GLOBAL_SUB8X8_USED
+    return 0;
+#endif  // !GLOBAL_SUB8X8_USED
 #if CONFIG_EXT_INTER
     if (mi->bmi[0].as_mode != ZEROMV || mi->bmi[1].as_mode != ZEROMV ||
         mi->bmi[2].as_mode != ZEROMV || mi->bmi[3].as_mode != ZEROMV ||