video_core/gpu: Remove use of the global system accessor

We can just make use of the reference member variable instead of
accessing the global system instance.

src/video_core/gpu.cpp

@@ -326,7 +326,7 @@ void GPU::ProcessSemaphoreTriggerMethod() {
         block.sequence = regs.semaphore_sequence;
         // TODO(Kmather73): Generate a real GPU timestamp and write it here instead of
         // CoreTiming
-        block.timestamp = Core::System::GetInstance().CoreTiming().GetTicks();
+        block.timestamp = system.CoreTiming().GetTicks();
         memory_manager->WriteBlock(regs.semaphore_address.SemaphoreAddress(), &block,
                                    sizeof(block));
     } else {