drm/i915: clean up backlight conditional build

I've always felt the backlight device conditional build has been all
backwards. Make it feel right.

Gently move things towards connector based stuff while at it.

There should be no functional changes.

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>