Make var-tx support YUV444 format

Properly scale the chroma block size accroding to the YUV format.

BUG=aomedia:522

Change-Id: I2c42763394eea6383f81443bf1098fe05f448bd5

av1/encoder/rdopt.c

diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index c1a3ede..1ca2e39 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -4475,7 +4475,8 @@
 
 #if CONFIG_CB4X4 && !CONFIG_CHROMA_2X2
   if (x->skip_chroma_rd) return is_cost_valid;
-  bsize = AOMMAX(BLOCK_8X8, bsize);
+  bsize = scale_chroma_bsize(mbmi->sb_type, xd->plane[1].subsampling_x,
+                             xd->plane[1].subsampling_y);
 #endif  // CONFIG_CB4X4 && !CONFIG_CHROMA_2X2
 
 #if CONFIG_EXT_TX && CONFIG_RECT_TX