Fix bugs in sb8x8 experiment/context prob update

Fix bugs occur in contextual partition probability update, when
sb8x8 is enabled.

Change-Id: I19e2cec8a54c2dafd2be2803bbfde7337a2ae45f

vp9/common/vp9_blockd.h

diff --git a/vp9/common/vp9_blockd.h b/vp9/common/vp9_blockd.h
index 2a72034..4f07afc 100644
--- a/vp9/common/vp9_blockd.h
+++ b/vp9/common/vp9_blockd.h
@@ -453,7 +453,7 @@
 static INLINE void update_partition_context(MACROBLOCKD *xd,
                                             BLOCK_SIZE_TYPE sb_type,
                                             BLOCK_SIZE_TYPE sb_size) {
-  int bsl = mi_width_log2(sb_size), bs = 1 << bsl;
+  int bsl = mi_width_log2(sb_size), bs;
   int bwl = mi_width_log2(sb_type);
   int bhl = mi_height_log2(sb_type);
   int boffset = mi_width_log2(BLOCK_SIZE_SB64X64) - bsl;
@@ -462,6 +462,12 @@
   if (bsl == 0)
     return;
 
+#if CONFIG_SB8X8
+  bs = 1 << (bsl - 1);
+#else
+  bs = 1 << bsl;
+#endif
+
   // update the partition context at the end notes. set partition bits
   // of block sizes larger than the current one to be one, and partition
   // bits of smaller block sizes to be zero.