drm/i915/dsi: fix dual-link DSI backlight and CABC ports for display 11+

The VBT dual-link DSI backlight and CABC still use ports A and C, both
in Bspec and code, while display 11+ DSI only supports ports A and
B. Assume port C actually means port B for display 11+ when parsing VBT.

Bspec: 20154
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/6476


Cc: stable@vger.kernel.org
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8c462718bcc7b36a83e09d0a5eef058b6bc8b1a2.1660664162.git.jani.nikula@intel.com