BacklightHelper: Allow overriding detected backlight devices via config

On some laptops, firmware backlight devices are detected and prioritized, but do not actually control the panel brightness. In such cases, the raw device is functional, but cannot be selected due to the current priority logic.

Since this behavior cannot be reliably detected in code (as brightness values may change in sysfs without affecting actual hardware), provide a configuration-based override as a workaround.

A new config file(/etc/powerdevil/backlighthelperrc) has been added with the following content:

[Backlight] 
PreferredBacklightDevices=/sys/class/backlight/amdgpu_bl2

The value is a list of absolute sysfs paths. If specified, these devices are used instead of auto-detected ones. Invalid entries (paths without brightness and max_brightness files) are ignored and fallback to auto-detection is preserved.

This is intended as a workaround for hardware/driver-specific issues.

BUG: 399646