drm/i915: Expect child dev size of 22 bytes for VBT < 106

My 830 has VBT version 105 with child device size of 22 bytes.
Let's assume that's correct and adjust our expectations.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1450110229-30450-9-git-send-email-ville.syrjala@linux.intel.com
Acked-by: Jani Nikula <jani.nikula@intel.com>