backends/drm: disable software brightness if there was ever a hardware brightness device assigned
Software brightness is very useful when the backlight really can't be controlled by us, but if it's just temporarily unavailable because of powerdevil not running, or DDC/CI having issues, then it's best if we don't darken the image in software on top of the backlight level still being set by the display.
(cherry picked from commit 4f18aae6)