[REGRESSION] Please revert 1191285ecb42 from 7.1.y

[Sergio Callegari <sergio.callegari@gmail.com>]

Hi again,

after having looked a bit more thoroughly into the matter, and having 
gone through the relevant commits that introduced and subsequently 
modified the AMD backlight brightness code paths, I would like to 
suggest reverting commit

1191285ecb4233e5bf907da7e9394d7999055566

from the 7.1 stable series.

On affected AMD systems this change introduces a user-visible brightness 
regression: values near the newly advertised maximum can wrap around to 
the minimum brightness. On an ASUS G14, for example, brightness 64532 
gives the expected maximum level, while 64533 immediately drops 
actual_brightness to 0; max_brightness is advertised as 65535.

The previous max - min limit therefore appears to have been significant 
to the existing conversion path. While commit 1191285 fixes an 
inconsistency between actual_brightness and max_brightness, it exposes a 
range that the current implementation does not handle correctly.

For stable kernels, reverting the commit seems to me the safest option: 
it restores the previous behaviour and avoids introducing a broader 
redesign into stable.

Longer term, I think the AMD backlight code would benefit from a proper 
rework of the brightness coordinate systems. In particular, it seems 
cleaner to expose a fixed logical DC range (e.g. 0..65535), map that 
symmetrically onto the firmware/hardware [min, max] range, and keep the 
firmware custom-curve 0..255 scale as a separate conversion where required.

That larger change could then be reviewed and tested upstream 
independently of the stable regression fix.

Thanks,
Sergio