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

main_wayland.cpp

@@ -589,7 +589,6 @@ int main(int argc, char * argv[])
     QCommandLineOption framebufferDeviceOption(QStringLiteral("fb-device"),
                                                i18n("The framebuffer device to render to."),
                                                QStringLiteral("fbdev"));
-    framebufferDeviceOption.setDefaultValue(QStringLiteral("/dev/fb0"));
     QCommandLineOption x11DisplayOption(QStringLiteral("x11-display"),
                                         i18n("The X11 Display to use in windowed mode on platform X11."),
                                         QStringLiteral("display"));