drm/i915/dsi: use VBT backlight and CABC port definitions directly

Drop the intermediate values stored in intel_dsi and use the VBT values
directly, now that they're conveniently stored in panel->vbt.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8b3765f1e1dc4d436b312016f72647e03ba49f94.1660664162.git.jani.nikula@intel.com