drm/i915: do not expose a dysfunctional backlight interface to userspace

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: Jani Nikula <jani.nikula@intel.com>
Tested-by: David Woodhouse <David.Woodhouse@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>