drm/i2c: silence ch7006.h and sil164.h kernel-doc warnings

Mark some members private to silence kernel-doc warnings, and add FIXME
comments.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ffc58be256d71e6a98eb9f13337add64458d3476.1709898638.git.jani.nikula@intel.com