hwc: Allow null fence on client taret buffer.

Client keeps track of GPU composition requests and sends -1 fd
if there is no need to re draw.

CRs-Fixed: 2695433
Change-Id: I310d848a32afecdaf52aa23c513565920fe21de2
diff --git a/composer/hwc_display.cpp b/composer/hwc_display.cpp
index bc36454..628c0f0 100644
--- a/composer/hwc_display.cpp
+++ b/composer/hwc_display.cpp
@@ -1212,8 +1212,7 @@
   }
 
   if (acquire_fence == nullptr) {
-    DLOGW("acquire_fence is zero");
-    return HWC2::Error::BadParameter;
+    DLOGV_IF(kTagClient, "Re-using cached buffer");
   }
 
   Layer *sdm_layer = client_target_->GetSDMLayer();