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
Add backlight function to linuxkpi.
Graphics drivers expose the backlight of the panel directly so allow them to use the backlight subsystem so
user can use backlight(8) to configure them.
Reviewed by: hselasky
Relnotes: yes
Differential Revision: The FreeBSD Foundation