Fix disabeling of autolock timeout

Summary:
When the "Lock Screen:" checkbox gets disabled the timeout spin box gets disabled too.
But the spin box is still enabled when the checkbox is disabled if the module is loaded.

Test Plan:
Disable screenlocking and restart module. The spin box should be disabled.
Before:
https://phabricator.kde.org/file/data/u6vxui546tvyluaoyo4l/PHID-FILE-b3uowaz5ar2xk7buwt6r/edit_before.png

After:
https://phabricator.kde.org/file/data/6cwds725zvhbtadcndxt/PHID-FILE-onnxln4uw4miuvcj7nbf/edit_after.png

Reviewers: mart, ervin

Reviewed By: ervin

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27063