drm/i915/bios: throw away struct old_child_dev_config

The old_child_dev_config struct is no longer needed except for its size;
replace with a macro. No functional change.

Cc: Animesh Manna <animesh.manna@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/4bfa6e45fecd33af797ec218635504ec8a09f788.1503600621.git.jani.nikula@intel.com