drm-backend: really reset/restart outputs of a failed commit

This reverts f843ba34 ("drm-backend: limit reset/restart to output of a failed commit") and actually solves the problem correctly.

The pending_state is no longer valid at this point, so it cannot be used to determine the outputs of the current commit. So only clear will_repaint when starting to repaint, so it can be used to determine which outputs of a device were actually repainted and use it to reset/restart those outputs.

Fixes #930 (closed).