drm/vc4: Shut down BO cache timer before teardown

In the Linux kernel, the following vulnerability has been resolved: drm/vc4: Shut down BO cache timer before teardown The BO cache timer callback schedules time_work, and time_work can rearm the timer through vc4_bo_cache_free_old(). vc4_bo_cache_destroy() deletes the timer and then cancels the work, which does not break that cycle: the work being cancelled can rearm the timer, and the timer then queues work again after teardown. Use timer_shutdown_sync() instead, so the timer cannot be rearmed and the cycle ends with cancel_work_sync().