drm/i915/bios: store child devices in a list

Using the array is getting clumsy. Make things a bit more dynamic.

Remove early returns on not having child devices when the end result
after "iterating" the empty list would be the same.

v3:
- use list_add_tail to not reverse the child device list (Ville)

v2:
- stick to previous naming of child devices (Ville)
- use kzalloc, handle failure
- initialize list head earlier to keep intel_bios_driver_remove() safe

Cc: Ville Syrjala <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/3e72da0b412354ed8be6719df55b0e0cc4caa61a.1573227240.git.jani.nikula@intel.com