drm/i915/bios: add missing __packed to structs used for reading vbt

This does not seem to make a difference for the structs in question, but
document the intent.

v2: also pack union child_device_config (Daniel)

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>