libweston: fbdev: Force the Framebuffer devices to be activated.

    
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 qemu
with two virtual QXL cards. This is a more precise way to activate
framebuffer devices with the ioctl