drm/i915/bios: abstract child device expected size

Add a function to return the expected child device size. Flip the if
ladder around and use the same versions as in documentation to make it
easier to verify. Return an error for unknown versions. No functional
changes.

v2: Move BUILD_BUG_ON() next to the expected sizes

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240226175854.287871-3-jani.nikula@intel.com


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