Make manual control compatible with redshift >= 1.12

Summary:
BUG: 395641

Redshift version >= 1.12 requires the -P option to clear the existing gamma ramps for one-shot mode. Without it the screen gets darker and darker until it is impossible to see anything.

Quoting from the redshift 1.12 manual page:

> -O TEMP One-shot manual mode (set color temperature).  Use this with the -P option to clear the existing gamma ramps before applying the new color temperature.
> -P Reset existing gamma ramps before applying new color effect.

Reviewers: martinkostolny, #plasma, mart

Reviewed By: #plasma, mart

Subscribers: mart, ltoscano, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23330