nvflinger: Use return value of Lock()

comex reported in #4424 that we were incorrectly discarding the return
value of Lock() which is correct.

src/core/hle/service/nvflinger/nvflinger.cpp

@@ -68,7 +68,7 @@ NVFlinger::NVFlinger(Core::System& system) : system(system) {
     // Schedule the screen composition events
     composition_event = Core::Timing::CreateEvent(
         "ScreenComposition", [this](u64, std::chrono::nanoseconds ns_late) {
-            Lock();
+            const auto guard = Lock();
             Compose();
 
             const auto ticks = std::chrono::nanoseconds{GetNextTicks()};

src/core/hle/service/nvflinger/nvflinger.h

@@ -86,7 +86,7 @@ public:
 
     s64 GetNextTicks() const;
 
-    std::unique_lock<std::mutex> Lock() {
+    std::unique_lock<std::mutex> Lock() const {
         return std::unique_lock{*guard};
     }