drm/i915/bios: Define VBT block 20 (OEM Customizable Modes) contents

Define the contents of VBT block 20 (OEM Customizable Modes).
Each entry is either 26 or 28 bytes, depending on the BDB version.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240503122449.27266-22-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>