Enable per transformed block zero coeffs forcing

This commit enables forcing all coefficients zero per transformed
block, when its rate-distortion cost is lower than regular coeff
quantization.

The overall performance improvement (including its parent patch on
calculating rd cost per transformed block) at speed 1:
derf:  0.298%
yt:    0.452%
hd:    0.741%
stdhd: 0.006%

Change-Id: I66005fe0fd7af192c3eba32e02fd6d77952accb5

vp9/encoder/vp9_encodeframe.c

@@ -390,6 +390,9 @@ static void update_state(VP9_COMP *cpi, PICK_MODE_CONTEXT *ctx,
   }
 
   x->skip = ctx->skip;
+  vpx_memcpy(x->zcoeff_blk[mbmi->tx_size], ctx->zcoeff_blk,
+             sizeof(ctx->zcoeff_blk));
+
   if (!output_enabled)
     return;
 
@@ -2744,7 +2747,6 @@ static void encode_superblock(VP9_COMP *cpi, TOKENEXTRA **t, int output_enabled,
     setup_pre_planes(xd, 1, second_ref_fb, mi_row, mi_col,
                      &xd->scale_factor[1]);
 
-
     vp9_build_inter_predictors_sb(xd, mi_row, mi_col, MAX(bsize, BLOCK_8X8));
   }