main_wayland: Don't hardcode /dev/fb0 anymore

Summary: Now that the framebuffer device is queried, don't assume that the framebuffer device is /dev/fb0

Test Plan:
Add /dev/fb1 to seat1, start kwin with the framebuffer backend on seat1, verify with the stdio/stderr output it selected /dev/fb1.
Make sure manual device selection still works

There is an issue right now where /dev/fb1 is all black, if it is both selected manually or automatically. I think I might need see if the VSCREENINFO or FSCREENINFO framebuffer settings are changing or something (I had to do something similar to this to Weston's fbdev backend as well)

If I do find a solution, I'll send one separate from this series

Reviewers: KWin, graesslin

Reviewed By: KWin, graesslin

Subscribers: zzag, rkflx, graesslin, kwin, KWin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D9557