ppu: Fix threads scheduler

*Set priority under a lock
*Fix yield command making threads going out of scheduler control by removing it from the queue (not a bug that affects compatibility)

rpcs3/Emu/Cell/lv2/lv2.cpp

@@ -1,4 +1,4 @@
-#include "stdafx.h"
+#include "stdafx.h"
 #include "Emu/System.h"
 
 #include "Emu/Cell/PPUFunction.h"
@@ -1063,7 +1063,15 @@ void lv2_obj::awake(cpu_thread& cpu, u32 prio)
 
 	std::lock_guard lock(g_mutex);
 
-	if (prio == -4)
+	if (prio < INT32_MAX)
+	{
+        // Priority set
+        if (static_cast<ppu_thread&>(cpu).prio.exchange(prio) == prio || !unqueue(g_ppu, &cpu))
+        {
+            return;
+        }
+	}
+	else if (prio == -4)
 	{
 		// Yield command
 		const u64 start_time = get_system_time();
@@ -1087,12 +1095,6 @@ void lv2_obj::awake(cpu_thread& cpu, u32 prio)
 		static_cast<ppu_thread&>(cpu).start_time = start_time;
 	}
 
-	if (prio < INT32_MAX && !unqueue(g_ppu, &cpu))
-	{
-		// Priority set
-		return;
-	}
-
 	// Emplace current thread
 	for (std::size_t i = 0; i <= g_ppu.size(); i++)
 	{

rpcs3/Emu/Cell/lv2/sys_ppu_thread.cpp

@@ -206,7 +206,7 @@ error_code sys_ppu_thread_set_priority(ppu_thread& ppu, u32 thread_id, s32 prio)
 
 	const auto thread = idm::check<named_thread<ppu_thread>>(thread_id, [&](ppu_thread& thread)
 	{
-		if (thread.prio != prio && thread.prio.exchange(prio) != prio)
+		if (thread.prio != prio)
 		{
 			lv2_obj::awake(thread, prio);
 		}