Lock and unlock __instanceLock__ directly for a Minor perf win in ASImageNode (#2797)

- Suggested by @appleguy in #2778

Author: nguyenhuy

AsyncDisplayKit/ASImageNode.mm

@@ -273,11 +273,9 @@ - (UIImage *)displayWithParameters:(id<NSObject> *)parameter isCancelled:(asdisp
     return nil;
   }
 
-  ASImageNodeDrawParameters drawParameter;
-  {
-    ASDN::MutexLocker l(__instanceLock__);
-    drawParameter = _drawParameter;
-  }
+  __instanceLock__.lock();
+  ASImageNodeDrawParameters drawParameter = _drawParameter;
+  __instanceLock__.unlock();
 
   CGRect drawParameterBounds       = drawParameter.bounds;
   BOOL forceUpscaling              = drawParameter.forceUpscaling;