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: {F7366118, size=full}

New: {F7366127, size=full}

Reviewers: #vdg, #breeze, ngraham

Reviewed By: #vdg, #breeze, ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

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