compositor-fbdev: set fb device info upon the first run.

This attempts to wake up secondary framebuffer devices
(/dev/fb1 and up) as usually these devices start powered off, and
the FBIOPUT_VSCREENINFO ioctl turns it on. This was tested on a
qemu system with the options:

-vga none -device VGA,id=video0 -device secondary-vga,id=video1 \
-device secondary-vga,id=video2

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