Make firstpass encode Y-only

The chroma planes are not used during the first pass encode,
but the vp9_encode_sb() function was operating on them anyway.
This was causing the use of uninitialized memory.

Change-Id: I5ebafcd3d5e34ed91a8336dad159b573995a939f

vp9/encoder/vp9_firstpass.c

diff --git a/vp9/encoder/vp9_firstpass.c b/vp9/encoder/vp9_firstpass.c
index 1e25a00..436c8d4 100644
--- a/vp9/encoder/vp9_firstpass.c
+++ b/vp9/encoder/vp9_firstpass.c
@@ -632,7 +632,7 @@
           vp9_build_inter_predictors_sby(xd, mb_row << 1,
                                          mb_col << 1,
                                          BLOCK_SIZE_MB16X16);
-          vp9_encode_sb(cm, x, BLOCK_SIZE_MB16X16);
+          vp9_encode_sby(cm, x, BLOCK_SIZE_MB16X16);
           sum_mvr += mv.as_mv.row;
           sum_mvr_abs += abs(mv.as_mv.row);
           sum_mvc += mv.as_mv.col;