udev: Add the ability to query the current framebuffer.

Summary:
This adds the ability for kwin's udev handler to detect a usable framebuffer device.
This is very similar to the function that queries the primary GPU.

Reviewers: KWin, graesslin

Reviewed By: KWin, graesslin

Subscribers: rkflx, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D9554