drm/i915: Pick the backlight controller based on VBT on ICP+

Use the second backlight controller on ICP+ if the VBT asks
us to do so.

On pre-MTP we also check the chicken bit to make sure the
pins have been correctly muxed by the firmware.

Cc: stable@vger.kernel.org
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/8016


Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230207064337.18697-4-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>
(cherry picked from commit b3377154)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>