drm/i915: Backlight setup requires connector so pass it as parameter

Get rid of saved int_lvds_connector and int_edp_connector in
drm_i915_private.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>