Solid-device-automounter/kcm: Improve width of columns

Summary: Make the "automatic login on..." columns thinner to give more room to the Peripheral column

Test Plan:
Before:

After:

Reviewers: ngraham, ervin, Plasma

Reviewed By: ngraham, ervin

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27053