This laptop has an BIOS option "Switchable Graphics". When the options is
selected, it behaves like a normal Optimus laptop. In this case, brightness
sysfs works since it's driven by Intel graphics.

When the options is deselected, the Intel graphics device disappears, Nvidia is
the only graphics device. The issue happens in this case.

Changing /sys/class/backlight/nv_backlight/brightness, the brightness level
remains the same - almost no backlight.

Using proprietary driver, brightness gets changed correctly by
/sys/class/backlight/nvidia_0/brightness.

No trace while using MMIOTRACE, under proprietary driver.