compositor-fbdev: detect the first fb device in the seat

This adds a function to detect the first framebuffer device in the
current seat. Instead of hardcoding /dev/fb0, detect the device
with udev, favoring the boot_vga device, and falling back to the
first framebuffer device in the seat if there is none. This is very
similar to what compositor-drm does to find display devices

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>