Point density: Workaround for possible race condition

There was possible race condition in the point density sampling caused
by access to the same data in particle system from sampling thread and
sampling initialization.

Could have happened when two different point density textures were using
same particle system

source/blender/render/intern/source/pointdensity.c

@@ -687,8 +687,6 @@ void RE_sample_point_density(Scene *scene, PointDensity *pd,
 
 	BLI_mutex_lock(&sample_mutex);
 	cache_pointdensity_ex(scene, pd, mat, mat, 1, 1);
-	BLI_mutex_unlock(&sample_mutex);
-
 	for (z = 0; z < resolution; ++z) {
 		for (y = 0; y < resolution; ++y) {
 			for (x = 0; x < resolution; ++x) {
@@ -711,4 +709,5 @@ void RE_sample_point_density(Scene *scene, PointDensity *pd,
 		}
 	}
 	free_pointdensity(pd);
+	BLI_mutex_unlock(&sample_mutex);
 }