backends/wayland: Fix a crash on shutdown

The wayland backend changes the dpms mode when the enabled state of the
output changes. This creates problems when destroying all outputs on
shut down. The wayland backend calls InputRedirection's methods, but
InputRedirection is already destroyed by that time.

While the crash can be fixed by guarding input() in createDpmsFilter()
with an if statement, changing dpms mode in updateEnablement() doesn't
seem like a good idea because Output changes unrelated states (enabled !=
dpms mode), so let's get rid of it.