Fix first-pass encoding test

Always initialize the mode_info with sb_type of BLOCK_SIZE_MB16X16
for the first-pass encoding test.

Change-Id: Ic86393eeef981bdd523a5b44cfac3f0b24c068b7

vp9/encoder/vp9_firstpass.c

@@ -522,6 +522,8 @@ void vp9_first_pass(VP9_COMP *cpi) {
       xd->plane[2].dst.buf = new_yv12->v_buffer + recon_uvoffset;
       xd->left_available = (mb_col != 0);
 
+      xd->mode_info_context->mbmi.sb_type = BLOCK_SIZE_MB16X16;
+
       // do intra 16x16 prediction
       this_error = vp9_encode_intra(cpi, x, use_dc_pred);