drm/i915: make backlight info per-connector

Move from dev_priv to connector->panel. We still don't allow multiple
sysfs interfaces, though.

There should be no functional changes, except for a slight reordering of
connector backlight and sysfs destroy calls. (This change happens now
that the backlight device is actually per-connector, even though the
destroy calls became per-connector earlier.)

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>