[effects/fade] Don't animate the logout screen

Summary:
The appearing of the logout screen is animated by both the Fade effect
and the Logout effect. The former should not animate the logout screen
because that's the job of the latter.

Test Plan:
Clicked the shut down button, the log out screen smoothly faded in.
Clicked the cancel button, the log out screen instantaneously disappeared.

Reviewers: #kwin, #plasma, davidedmundson

Reviewed By: #kwin, #plasma, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D14582