drm/i915: Move panel's backlight setup next to panel init

drm/i915: Move panel's backlight setup next to panel init

Currently setting up the backlight for a panel is sometimes done
together with initialising the panel, and sometimes after the connector
is registered. The backlight setup does not depend upon connector
registration (i.e. access to sysfs/debugfs and the kobject hierachy) so
perform it consistently just after panel initialisation.

Note the discrepancy here as destroying the panel is done during
connector unregistration...

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1466773227-7994-1-git-send-email-chris@chris-wilson.co.uk