MaxwellDMA/KeplerCopy: Downgrade DMA log message to Trace.

This log was just to know which games used DMA. It's no longer 
important.

src/video_core/engines/maxwell_dma.cpp

@@ -38,7 +38,7 @@ void MaxwellDMA::CallMethod(const GPU::MethodCall& method_call) {
 }
 
 void MaxwellDMA::HandleCopy() {
-    LOG_WARNING(HW_GPU, "Requested a DMA copy");
+    LOG_TRACE(HW_GPU, "Requested a DMA copy");
 
     const GPUVAddr source = regs.src_address.Address();
     const GPUVAddr dest = regs.dst_address.Address();