Reset show existing frame type for regular ARFs only

BUG=aomedia:3079

Change-Id: I66f78229ad7333391660799c208a369960b29797
diff --git a/av1/encoder/encode_strategy.c b/av1/encoder/encode_strategy.c
index 7d1358c..3663406 100644
--- a/av1/encoder/encode_strategy.c
+++ b/av1/encoder/encode_strategy.c
@@ -971,7 +971,8 @@
                                         source_buffer->metadata);
     }
     // Currently INTNL_ARF_UPDATE only do show_existing.
-    if (update_type == ARF_UPDATE && !cpi->no_show_fwd_kf) {
+    if (update_type == ARF_UPDATE &&
+        gf_group->frame_type[cpi->gf_frame_index] != KEY_FRAME) {
       cpi->show_existing_alt_ref = show_existing_alt_ref;
     }
   }