hwc2: Use current frames retire fence only for the primary

Fixes regression with virtual displays introduced with
d369ccd4bc54a465dbc4693706360a1416f674e7
Change-Id: Id2fdcb01abd47b43ddad149d617828413422ecf3
diff --git a/msm8996/sdm/libs/hwc2/hwc_display.cpp b/msm8996/sdm/libs/hwc2/hwc_display.cpp
index 5cbf0b8..edd5bf2 100644
--- a/msm8996/sdm/libs/hwc2/hwc_display.cpp
+++ b/msm8996/sdm/libs/hwc2/hwc_display.cpp
@@ -891,7 +891,9 @@
       layer_stack_.retire_fence_fd = -1;
     }
     stored_retire_fence_ = layer_stack_.retire_fence_fd;
-    *out_retire_fence = layer_stack_.retire_fence_fd;
+    if (id_ == HWC_DISPLAY_PRIMARY) {
+      *out_retire_fence = layer_stack_.retire_fence_fd;
+    }
 
     if (dump_frame_count_) {
       dump_frame_count_--;