Display brightness OSD even when min and max is reached

Summary:
When the brightness is already at the maximum (or minimum) value,
hitting the keyboard shortcut to increase (resp. decrease) it does
not bring up the OSD, there is no visual feedback.
This is not consistent either with the volume which handles this
correctly.

BUG: 387284
FIXED-IN: 5.14.0

Reviewers: ngraham, romangg

Reviewed By: ngraham, romangg

Subscribers: romangg, ngraham, broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D13030