drm/i915: Move backlight registration to connector registration

Currently the backlight is being registered in the load phase (before
the display and its objects are registered). Move the backlight
registration into the analogous phase by performing it from the
connector registration, just after its creation.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1466773227-7994-3-git-send-email-chris@chris-wilson.co.uk