Don't duplicate connect to wl_display_flush

Summary:
When using the QPA's wl_display, we don't need to also flush when the
event dispatcher blocks, as Qt's QPA already does this.

This also fixes a warning on startup about a null parameter when our QPT
creates a connectionThread before the eventDispatcher is created.

Test Plan:
Confirmed the Qt wayland QPA has the same connect to the event dispatcher

Ran app:
No more warning
Still had correct window decorations

Subscribers: plasma-devel, Frameworks

Tags: Plasma on Wayland, Frameworks

Differential Revision: https://phabricator.kde.org/D8284