vp9_prob_diff_update_savings_search_model: quiet conv warnings

under Visual Studio:
Warning C4244 '=': conversion from 'int64_t' to 'vpx_prob', possible loss of
data

after:
ea042a676 vp9 encoder: fix integer overflows

'newp' has already been range checked earlier in the loop so the cast won't
have any unexpected results

Change-Id: Ic10877db2c0633d53fffdf8852d5095403c23a02
diff --git a/vp9/encoder/vp9_subexp.c b/vp9/encoder/vp9_subexp.c
index 661294b..3953253 100644
--- a/vp9/encoder/vp9_subexp.c
+++ b/vp9/encoder/vp9_subexp.c
@@ -163,14 +163,14 @@
     for (newp = *bestp; (newp - oldp) * step_sign < 0; newp += step) {
       if (newp < 1 || newp > 255) continue;
       newplist = vp9_pareto8_full[newp - 1];
-      new_b = cost_branch256(ct + 2 * PIVOT_NODE, newp);
+      new_b = cost_branch256(ct + 2 * PIVOT_NODE, (vpx_prob)newp);
       for (i = UNCONSTRAINED_NODES; i < ENTROPY_NODES; ++i)
         new_b += cost_branch256(ct + 2 * i, newplist[i - UNCONSTRAINED_NODES]);
-      update_b = prob_diff_update_cost(newp, oldp) + upd_cost;
+      update_b = prob_diff_update_cost((vpx_prob)newp, oldp) + upd_cost;
       savings = old_b - new_b - update_b;
       if (savings > bestsavings) {
         bestsavings = savings;
-        bestnewp = newp;
+        bestnewp = (vpx_prob)newp;
       }
     }
   }