drm/i915: Move backlight unregistration to connector unregistration

Currently the backlight is being unregistered in the unload phase (after
the display and its objects are unregistered). Move the backlight
unregistration into the analogous phase by performing it from the
connector unregistration, just prior to its deletion.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1466160034-12173-3-git-send-email-chris@chris-wilson.co.uk