Unify fwd key frame related check conditions

BUG=aomedia:3079

Change-Id: Ic5b723d680f3c5d98016fc7752f5bea85697ad15
diff --git a/av1/encoder/encode_strategy.c b/av1/encoder/encode_strategy.c
index 3663406..eecce2e 100644
--- a/av1/encoder/encode_strategy.c
+++ b/av1/encoder/encode_strategy.c
@@ -1486,11 +1486,9 @@
 #endif
 
   if (!is_stat_generation_stage(cpi)) {
-    // If this is a forward keyframe, mark as a show_existing_frame
-    // TODO(bohanli): find a consistent condition for fwd keyframes
-    if (oxcf->kf_cfg.fwd_kf_enabled &&
-        gf_group->update_type[cpi->gf_frame_index] == OVERLAY_UPDATE &&
-        cpi->rc.frames_to_key == 0) {
+    // TODO(jingning): fwd key frame always uses show existing frame?
+    if (gf_group->update_type[cpi->gf_frame_index] == OVERLAY_UPDATE &&
+        gf_group->refbuf_state[cpi->gf_frame_index] == REFBUF_RESET) {
       frame_params.show_existing_frame = 1;
     } else {
       frame_params.show_existing_frame =