vp9-aqmode=3: Limit on period for golden update.

For aqmode=3, golden period update is set based on period of cyclic refresh.
Put a limit on max golden period update, for now set to 40.
And fix comment.

Change-Id: Icb61dd87c796cce2a5f5f7331c6a129540994696

vp9/encoder/vp9_aq_cyclicrefresh.c

@@ -321,11 +321,11 @@ void vp9_cyclic_refresh_postencode(VP9_COMP *const cpi) {
 void vp9_cyclic_refresh_set_golden_update(VP9_COMP *const cpi) {
   RATE_CONTROL *const rc = &cpi->rc;
   CYCLIC_REFRESH *const cr = cpi->cyclic_refresh;
-  // Set minimum gf_interval for GF update to a multiple (== 2) of refresh
-  // period. Depending on past encoding stats, GF flag may be reset and update
-  // may not occur until next baseline_gf_interval.
+  // Set minimum gf_interval for GF update to a multiple of the refresh period,
+  // with some max limit. Depending on past encoding stats, GF flag may be
+  // reset and update may not occur until next baseline_gf_interval.
   if (cr->percent_refresh > 0)
-    rc->baseline_gf_interval = 4 * (100 / cr->percent_refresh);
+    rc->baseline_gf_interval = VPXMIN(4 * (100 / cr->percent_refresh), 40);
   else
     rc->baseline_gf_interval = 40;
 }