Use KFormat also in performance preferences

Summary:
Format the amount of available RAM using KFormat, but keeping the same
unit, and scale (since the controls there refer to specific ones).

Test Plan:
The amount of RAM is shown in KiB as before, although
formatted according to KFormat & locale.

Reviewers: Krita, rempt

Reviewed By: Krita, rempt

Tags: Krita

Differential Revision: https://phabricator.kde.org/D13775

(cherry picked from commit ae2463497d42bb10d7982f8c699a36c07c066ee1)