Fix kscreenlocker_greet --switchuser

Summary:
Years ago support was added for the lockscreen to default into switch user
mode. The goal being to simplify one more task from ksmserver.

It got postponed because it clashed with a change Kai made at the time.
Now I'm restoring that.

The code broke in the lockscreen rewrite.

Test Plan:
Called the switchUser DBus method
Lockscreen spawned on the right page
Locked screen normally, got the normal page

Reviewers: #plasma, mart

Reviewed By: #plasma, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D15185