Fix checking initial screen aspect ratio

The first time QWaylandScreen gets an output_geometry() event its
screen() may not have been initialized yet, so manually check
if the output is portrait or landscape.

Change-Id: I1d58daee7a3f168a881a1e2d21956c52c09818fc
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>