[CFL] Compute alpha costs using av1_cost_symbol

Use the uniform way to compue the cost of symbols in AV1.

Results on Subset1 (compared to 8a516a8f with CfL enabled)

   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-0.0357 | -0.0854 |  0.0305 |  -0.0422 | -0.0097 | -0.0171 |    -0.1042

Change-Id: Ie908fc7d20c480634002c78027b070223b3ea96d

av1/encoder/encodemb.c

diff --git a/av1/encoder/encodemb.c b/av1/encoder/encodemb.c
index 1692005..a9e2f2a 100644
--- a/av1/encoder/encodemb.c
+++ b/av1/encoder/encodemb.c
@@ -1492,18 +1492,17 @@
 static inline void cfl_update_costs(CFL_CTX *cfl, FRAME_CONTEXT *ec_ctx) {
   assert(ec_ctx->cfl_alpha_cdf[CFL_ALPHABET_SIZE - 1] ==
          AOM_ICDF(CDF_PROB_TOP));
-  const int prob_den = CDF_PROB_TOP;
 
-  int prob_num = AOM_ICDF(ec_ctx->cfl_alpha_cdf[0]);
-  cfl->costs[0] = av1_cost_zero(get_prob(prob_num, prob_den));
+  aom_cdf_prob prev_cdf = 0;
 
-  for (int c = 1; c < CFL_ALPHABET_SIZE; c++) {
-    int sign_bit_cost = (cfl_alpha_codes[c][CFL_PRED_U] != 0) +
-                        (cfl_alpha_codes[c][CFL_PRED_V] != 0);
-    prob_num = AOM_ICDF(ec_ctx->cfl_alpha_cdf[c]) -
-               AOM_ICDF(ec_ctx->cfl_alpha_cdf[c - 1]);
-    cfl->costs[c] = av1_cost_zero(get_prob(prob_num, prob_den)) +
-                    av1_cost_literal(sign_bit_cost);
+  for (int c = 0; c < CFL_ALPHABET_SIZE; c++) {
+    const int sign_bit_cost = (cfl_alpha_codes[c][CFL_PRED_U] != 0) +
+                              (cfl_alpha_codes[c][CFL_PRED_V] != 0);
+
+    aom_cdf_prob prob = AOM_ICDF(ec_ctx->cfl_alpha_cdf[c]) - prev_cdf;
+    prev_cdf = AOM_ICDF(ec_ctx->cfl_alpha_cdf[c]);
+
+    cfl->costs[c] = av1_cost_symbol(prob) + av1_cost_literal(sign_bit_cost);
   }
 }