Add function to fetch chroma block size

This helps reduce code and logic complexity.

Change-Id: I2456e464025e18ccd57a018a1d5f424e7e08ff0b
diff --git a/av1/encoder/rd.h b/av1/encoder/rd.h
index cc1484f..637cef2 100644
--- a/av1/encoder/rd.h
+++ b/av1/encoder/rd.h
@@ -235,7 +235,7 @@
 
 void av1_set_mvcost(MACROBLOCK *x, int ref, int ref_mv_idx);
 
-void av1_get_entropy_contexts(BLOCK_SIZE bsize,
+void av1_get_entropy_contexts(BLOCK_SIZE plane_bsize,
                               const struct macroblockd_plane *pd,
                               ENTROPY_CONTEXT t_above[MAX_MIB_SIZE],
                               ENTROPY_CONTEXT t_left[MAX_MIB_SIZE]);