drm/i915: make asle notifications update backlight on all connectors

ALthough usually there's only one connector that supports backlight,
this also finds the correct connector. Before, we only updated the
connector on pipe A, which might not be the one with backlight. (This
only made a difference on BYT.)

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>