drm/i915/bios: Define VBT block 55 (RGB Palette Table) contents

Define the contents of VBT block 55 (RGB Palette Table).

Note that I've not actually seen any real world VBTs with this
block.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240503122449.27266-31-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>