Previously intel_panel_setup_backlight() would create a sysfs backlight
interface with max brightness of 1 if it was unable to figure out the max
backlight brightness. This rendered the backlight interface useless.

Do not create a dysfunctional backlight interface to begin with.
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Tested-by: NDavid Woodhouse <David.Woodhouse@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>