Fix show existing frame for subgop

Change-Id: If3246e9a6234c046453dec839dd2b88d854a974b

av1/encoder/encode_strategy.c

diff --git a/av1/encoder/encode_strategy.c b/av1/encoder/encode_strategy.c
index 65cac89..900ea1d 100644
--- a/av1/encoder/encode_strategy.c
+++ b/av1/encoder/encode_strategy.c
@@ -1212,6 +1212,9 @@
         get_frame_update_type(&cpi->gf_group) == KFFLT_UPDATE) {
       cpi->show_existing_alt_ref = show_existing_alt_ref;
     }
+  } else if (get_frame_update_type(&cpi->gf_group) == ARF_UPDATE ||
+             get_frame_update_type(&cpi->gf_group) == KFFLT_UPDATE) {
+    cpi->show_existing_alt_ref = 1;
   }
 
   // perform tpl after filtering