BrightnessUtils: Conditionally disable gamma conversion for brightness slider

Backlight values are already gamma corrected.

Change-Id: I83b38a1334b8df156e4e53b06243c48d565be5d5
Co-authored-by: minaripenguin <minaripenguin@users.noreply.github.com>
Co-authored-by: Cosmin Tanislav <demonsingur@gmail.com>