st/egl: update fbdev backend

st/egl: update fbdev backend

Considering fbdev as an in-kernel window system,

 - opening a device opens a connection
 - there is only one window: the framebuffer
 - fb_var_screeninfo decides window position, size, and even color format
 - there is no pixmap

Now EGL is built on top of this window system.  So we should have

 - the fd as the handle of the native display
 - reject all but one native window: NULL
 - no pixmap support

modeset support is still around, but it should be removed soon.