drm/i915: hsw backlight registers need transcoder instead of pipe

v2: Make TRANSCODER_EDP handling more explicit. (Imre)

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>