Comment # 11 on bug 1209296 from Egbert van der Haring

just (16 march) installed a new version of the nouveau driver
(23.0.0-pm1599.347.1) but that did NOT restore the backlight control
functionality. 

What did change though, is that on shutdown / reboot a stop job is running
again. This has long been the case in functioning (wrt backlight control)
kernels, and disappeared in the non-functioning kernels. So it seems that
something did change with the new nouveau driver, except it does not have the
desired effect.