drm/i915: Bump VBT version for expected child dev size check

The most modern VBT I've observed in the wild is version 250.
The child dev size hasn't changed since version 216, so bump
the version number in the expected child dev size check.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230306154419.23207-1-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>