drm/i915/bios: abstract child device size check

Separate the child device size check to a separate function for
clarity. No functional changes.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240226175854.287871-2-jani.nikula@intel.com


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