[dimdisplay] Do not change screen brightness on invalid values

Authored by anthonyfieroni on Dec 30 2018, 7:24 AM.

Description

[dimdisplay] Do not change screen brightness on invalid values

Differential Revision: https://phabricator.kde.org/D16918

Signed-off-by: Anthony Fieroni <bvbfan@abv.bg>