drm/i915/bios: bump expected child device size

VBT versions since 256 have an extra byte for EFP index.

v2: Update BUILD_BUG_ON() (Matt)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240226175854.287871-1-jani.nikula@intel.com


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