shell: Set the DPMS state if the screensaver fails to launch or dies

shell: Set the DPMS state if the screensaver fails to launch or dies

The lock hook in desktop-shell only changes the DPMS state the second
time it is called, because during the first time it launches the
screensaver and wakes the compositor again when the screensaver surface
is configured. However, if the screensaver fails to launch, the output
is left in an enabled state, even thought there's no content being
displayed on the screen.

Fix this by disabling the outputs when the screensaver dies if the
shell is still locked.

https://bugs.freedesktop.org/show_bug.cgi?id=60084