avoid duplicate desktopviews on first startup

Summary:
on first startup all the desktop views already exist after the
script has been executed, so check the screen doesn't already have
a view for that screen before creating one

BUG:368891

Test Plan: tested both first and subsequent startups, one desktopview per screen created, containment config dialog coming up (and only one of them)

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D2885