LinuxKPI: Implement backlight_enable and backlight_disable functions

For now, disable backlight if brightness level is set to 0.
In the future we may implement separate knob in backlight(8).

Required by drm-kmod v5.6

Reviewed by: hselasky, manu
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D32165