Don't animate IconItem in software mode

Summary:
IconItem uses a gorgeous shader to fade between two states. This doesn't
work with software rendering.

Given software rendering is a second class citizen, just skip the
animation.

Test Plan:
Hovered over kickoff.
Went from one to the other, without temporarily disappearing

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D8014