drm/i915/bios: add VBT swing bit to child device definition

New bit to look in another BDB block for more. No functional changes.

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/7ab46e3f53fd2c12cb60b9eabbebb65b27004a9e.1559308269.git.jani.nikula@intel.com