[Daniel Vetter]
acpi_backlight=vendor should disable the acpi backlights (you can check that in /sys/class/backlight), which would mean that userspace should fall back to intel backlight driver. Can you please check whether that one works even without the invert_brightness knob? I.e. not just whether you'll end up with a black screen, but whether adjusting the backlight also works correctly.
Adjusting backlight using the laptop keys for this work in KDE when I boot with acpi_backlight=vendor and without invert_brightness=1. I am not sure if the buttons work the right way, though (never seen which should increase or decrease the brighness), so it might be inverted.
Is it possible to tell acpi_backlight to invert the brighness setting? Would it affect i915 too?