drm/i915/bios: Define the (obsolete) backlight i2c VBT stuff

drm/i915/bios: Define the (obsolete) backlight i2c VBT stuff

The VBT backlight entries include various fields for i2c
controlled backlights. These have been obsoleted at some
unknown point in time, but let's define them anyway so that
we have a full picture of what has been in there.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240321161806.31714-5-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>