The brightness is fixed at 100%. This is a limation in the "native graphics"
code, but I suspect it will added in the future.
("native graphics" or "native gfx init" or "native vga init" is a term used
in coreboot)
(it is a free VBIOS / VGA ROM replacement)
A VBIOS inits the GPU and controls things like backlight, before the OS
boots. It used to be that the only option for this was to extract the nonfree
VBIOS from the original firmware and put that in coreboot and have SeaBIOS
run it. Now there is a free replacement, which is what you have but the
replacement does have a few limitations currently.
