Cycles: Pass requested tile size to the device via device task

This is currently unused but crucial for things like calculating amount of
device memory required to deal with the tasks.

Maybe not really best place to store it, but consider it good enough for now.

intern/cycles/render/session.cpp

@@ -859,6 +859,7 @@ void Session::path_trace()
 	task.update_progress_sample = function_bind(&Session::update_progress_sample, this);
 	task.need_finish_queue = params.progressive_refine;
 	task.integrator_branched = scene->integrator->method == Integrator::BRANCHED_PATH;
+	task.requested_tile_size = params.tile_size;
 
 	device->task_add(task);
 }