Offset default probs in motion_var to account for cb4x4 mode

Offset the default probability set in motion_var to account for
the added block sizes in cb4x4 mode.

Change-Id: I18d90fda1678fad2fc738036e0d9caff6ac894b7

av1/common/entropymode.c

@@ -559,6 +559,9 @@ const aom_tree_index av1_motion_mode_tree[TREE_SIZE(MOTION_MODES)] = {
 };
 static const aom_prob default_motion_mode_prob[BLOCK_SIZES]
                                               [MOTION_MODES - 1] = {
+#if CONFIG_CB4X4
+                                                { 255 }, { 255 }, { 255 },
+#endif
                                                 { 255 }, { 255 }, { 255 },
                                                 { 151 }, { 153 }, { 144 },
                                                 { 178 }, { 165 }, { 160 },