hwc: Do not overwrite retireFenceFd if already set

* Do not overwrite retireFenceFd of displaylist in
hwc_set_virtual if it is already set

Change-Id: I785618bf3b266dc6fa8e881677df6e0849870ae7
diff --git a/libhwcomposer/hwc.cpp b/libhwcomposer/hwc.cpp
index ea6711b..113e916 100644
--- a/libhwcomposer/hwc.cpp
+++ b/libhwcomposer/hwc.cpp
@@ -576,7 +576,7 @@
 
     closeAcquireFds(list);
 
-    if (list && !ctx->mVirtualonExtActive) {
+    if (list && !ctx->mVirtualonExtActive && (list->retireFenceFd < 0) ) {
         // SF assumes HWC waits for the acquire fence and returns a new fence
         // that signals when we're done. Since we don't wait, and also don't
         // touch the buffer, we can just handle the acquire fence back to SF