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:

{F8045703}

After:

{F8045704}

Reviewers: ngraham, ervin, #plasma

Reviewed By: ngraham, ervin

Subscribers: plasma-devel

Tags: #plasma

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