drm/i915: register backlight device also when backlight class is a module

Ville and I were wondering why his laptop was missing the
intel_backlight sysfs interface. Turns out we never register it when
CONFIG_BACKLIGHT_CLASS_DEVICE=m. This has been broken ever since the
i915 native backlight interface was added.

CC: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>