drm/i915: fix non-DP-D eDP backlight cleanup and module reload

Backlight is initialized for eDP, but cleaned up only for eDP on DP-D
port. This leaves behind a dangling backlight interface on module unload on
machines that have eDP connected to something other than DP-D, and breaks
the backlight interface for subsequent module reloads. Fix the cleanup, and
thus module reload on affected machines.

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