restore dpms state on drm resume

Created by: Ongy

If there is no current mode, set outputs to dpms off in drm resume. Sets current mode to null on disable to ensure this can be checked.

Fixes https://github.com/swaywm/wlroots/issues/675

Testplan: Disable an output in the config (or by any means) and change tty away and back to rootston again.