drm/i915: Do intel_panel_destroy_backlight() later

drm/i915: Do intel_panel_destroy_backlight() later

Currently we destroy the backlight during connector unregistration.
That means the final modeset performed by drm_atomic_helper_shutdown()
will leave the backlight on. We don't want that so let's just move
intel_panel_destroy_backlight() into intel_panel_fini() which gets
called during connector destuction.

We still unregister the user visible backlight device during connector
unregistration.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181008134641.24868-1-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106386