[platforms/fbdev] Port to AbstractOutput

Summary:
To homogenize our backends and as another step to remove the Screens class
use the AbstractOutput class in the framebuffer backend.

Test Plan: Manually on VT enforcing the framebuffer backend.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D19199