The "disable semaphores" patch: http://cgit.freedesktop.org/~agd5f/linux/commit/?h=drm-fixes-4.1&id=013ead48a843442e63b9426e3bd5df18ca5d054a appears to stop the lock-ups from happening. It also appears to prevent one having multiple vdpau-enabled vlc sessions from working at the same time, and there are some issues with videos of different resolutions sometimes showing a black screen when their resolution is different from the previous video played. Playing a few different videos with vlc appears to reset things so that a video that previously showed a black screen plays fine again.