I have some recollection that someone else reported a similar issue with a
GP10x laptop GPU -- the backlight control wasn't working.

Perhaps this is similar. Or perhaps we need to explicitly turn the PANEL_POWER
back on (we do it when initially detecting the panel, but never touch it again
in nouveau_connector.c)

Or perhaps NV_DEVICE_INFO_V0_PASCAL is missing from the switch in
nouveau_backlight.c (it is), although as I was saying, someone was having
trouble with it... see bug #100446.