libgui: Don't assign handle to NULL after free

This reverts c784dfc39f for exynos4 devices
with Mali 400 GPUs, which causes a fatal signal (SIGSEGV) and death of
the graphics subsystem

Change-Id: I6dbf8f8664fca01baf63fece7c64016609fe3e1c

libs/ui/GraphicBuffer.cpp

@@ -112,7 +112,11 @@ void GraphicBuffer::free_handle()
         GraphicBufferAllocator& allocator(GraphicBufferAllocator::get());
         allocator.free(handle);
     }
+
+#ifndef EXYNOS4_ENHANCEMENTS
     handle = NULL;
+#endif
+
     mWrappedBuffer = 0;
 }