revert changes made in c4a0ad1 as it causes iteration time increase

Author: ftian1

src/caffe/syncedmem.cpp

@@ -263,10 +263,8 @@ void SyncedMemory::set_prv_descriptor(shared_ptr<PrvMemDescr> descriptor,
   } else {
     if ((head_ != HEAD_AT_PRV) && same_data)
       head_ = SYNCED_PRV;
-    else if (descriptor->prv_ptr())
-      head_ = HEAD_AT_PRV;
     else
-      head_ = HEAD_AT_CPU;
+      head_ = HEAD_AT_PRV;
   }
 
   prv_descriptor_ = descriptor;