video_core: Block in WaitFence.

This function is called rarely and blocks quite often for a long time.
So don't waste power and let the CPU sleep.

This might also increase the performance as the other cores might be allowed to clock higher.

src/video_core/gpu.cpp

@@ -66,19 +66,20 @@ const DmaPusher& GPU::DmaPusher() const {
     return *dma_pusher;
 }
 
-void GPU::WaitFence(u32 syncpoint_id, u32 value) const {
+void GPU::WaitFence(u32 syncpoint_id, u32 value) {
     // Synced GPU, is always in sync
     if (!is_async) {
         return;
     }
     MICROPROFILE_SCOPE(GPU_wait);
-    while (syncpoints[syncpoint_id].load(std::memory_order_relaxed) < value) {
-    }
+    std::unique_lock lock{sync_mutex};
+    sync_cv.wait(lock, [=]() { return syncpoints[syncpoint_id].load() >= value; });
 }
 
 void GPU::IncrementSyncPoint(const u32 syncpoint_id) {
     syncpoints[syncpoint_id]++;
     std::lock_guard lock{sync_mutex};
+    sync_cv.notify_all();
     if (!syncpt_interrupts[syncpoint_id].empty()) {
         u32 value = syncpoints[syncpoint_id].load();
         auto it = syncpt_interrupts[syncpoint_id].begin();