Make renderDialGroove() area match the maximum renderDialContents() area

Summary: It bothered me that the groove for dials doesn't match the maximum area that the contents can use.

Test Plan:
Old:

New:

Reviewers: VDG, Breeze, ngraham

Reviewed By: VDG, Breeze, ngraham

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24008