Fix whether a panel is supposed to have a strut in ShellClient

Summary:
The PanelBehavior was incorrectly mapped to hasStrut resulting in too
many modes creating a strut for the panel.

CCBUG: 368499

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D2788