[platforms/fbdev] Create output device

Summary: Create output device in framebuffer backend.

Test Plan: On VT started dbus-run-session kwin_wayland --framebuffer --xwayland.

Reviewers: KWin

Subscribers: kwin

Tags: KWin

Maniphest Tasks: T11459

Differential Revision: https://phabricator.kde.org/D23478