[lv_map_multi] simplify update_cdf

remove tmp0 in update_cdf due to the use of EC_MIN_PROB introduced by
Thomas Davies.

further changes to update_cdf include:
1. Start the rate at 3+get_msb(nsymbs) and increase the rate by one at
counts 16 and 32.
2. Check if tmp is less than cdf[i] to avoid shifting a negative number.

Change-Id: I5088ebd450d6e57ec6c3e92bb2f47a078489b947

av1/encoder/cost.c

diff --git a/av1/encoder/cost.c b/av1/encoder/cost.c
index 240a16c..a983105 100644
--- a/av1/encoder/cost.c
+++ b/av1/encoder/cost.c
@@ -71,7 +71,12 @@
   int i;
   aom_cdf_prob prev_cdf = 0;
   for (i = 0;; ++i) {
+#if CONFIG_LV_MAP_MULTI
     aom_cdf_prob p15 = AOM_ICDF(cdf[i]) - prev_cdf;
+    p15 = (p15 < EC_MIN_PROB) ? EC_MIN_PROB : p15;
+#else
+    aom_cdf_prob p15 = AOM_ICDF(cdf[i]) - prev_cdf;
+#endif
     prev_cdf = AOM_ICDF(cdf[i]);
 
     if (inv_map)