fix a VP8 build issue

coef_counts is now in cpi->mb, instead of cpi. The commit corrected the
mis-use and enable succefual build.

Change-Id: I0e77909d34571cfd2560c66b46b1f8fa0cd1a6b4

vp8/encoder/bitstream.c

diff --git a/vp8/encoder/bitstream.c b/vp8/encoder/bitstream.c
index 8ca4f5f..78e54e2 100644
--- a/vp8/encoder/bitstream.c
+++ b/vp8/encoder/bitstream.c
@@ -1062,7 +1062,7 @@
     if (cpi->common.frame_type == KEY_FRAME)
     {
         /* Reset to default counts/probabilities at key frames */
-        vp8_copy(cpi->coef_counts, default_coef_counts);
+        vp8_copy(cpi->mb.coef_counts, default_coef_counts);
     }
 
     if (cpi->oxcf.error_resilient_mode & VPX_ERROR_RESILIENT_PARTITIONS)