Use the same icon mode calculation for comboboxes as for buttons

Summary: When hovering a focused combobox the icon was in Selected state resulting in a wrong color.

Test Plan: Hover over a focused combobox that has an icon

Reviewers: broulik, Breeze, ndavis

Reviewed By: Breeze, ndavis

Subscribers: ndavis, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D28908