[GSP::GPU] Correct setBufferSwap (also add a panic until a test case appears)

src/core/services/gsp_gpu.cpp

@@ -270,7 +270,13 @@ void GPUService::setBufferSwap(u32 messagePointer) {
 	info.stride = mem.read32(messagePointer + 20);
 	info.format = mem.read32(messagePointer + 24);
 	info.displayFb = mem.read32(messagePointer + 28);  // Selects either framebuffer A or B
+
+	log("GSP::GPU::SetBufferSwap\n");
+	Helpers::panic("Untested GSP::GPU::SetBufferSwap call");
+
 	setBufferSwapImpl(screenId, info);
+	mem.write32(messagePointer, IPC::responseHeader(0x05, 1, 0));
+	mem.write32(messagePointer + 4, Result::Success);
 }
 
 // Seems to also be completely undocumented