Backlight control broken on UM325 (OLED) on 5.15 (bisected)

[Samuel Čavoj]

Hello,

the backlight control no longer works on my ASUS UM325 (Ryzen 5700U)
OLED laptop. I have bisected the breakage to commit 7fd13baeb7a3a48.

    commit 7fd13baeb7a3a48cae12c36c52f06bf4e9e7d728 (HEAD, refs/bisect/bad)
    Author: Alex Deucher <alexander.deucher at amd.com>
    Date:   Thu Jul 8 16:31:10 2021 -0400
     
        drm/amdgpu/display: add support for multiple backlights
        
        On platforms that support multiple backlights, register
        each one separately.  This lets us manage them independently
        rather than registering a single backlight and applying the
        same settings to both.
        
        v2: fix typo:
        Reported-by: kernel test robot <lkp at intel.com>
        
        Reviewed-by: Roman Li <Roman.Li at amd.com>
        Signed-off-by: Alex Deucher <alexander.deucher at amd.com>

I have encountered another user with the same issue on reddit[0].

The node in /sys/class/backlight exists, writing to it just does
nothing. I would be glad to help debugging the issue. Thank you very
much.

Regards,
Samuel Čavoj

[0]: https://www.reddit.com/r/AMDLaptops/comments/qst0fm/after_updating_to_linux_515_my_brightness/