drm/i915: Allow 27 bytes child_dev for VBT <109

My 85x has VBT version 108 which has a child dev size of 27 bytes.
Let's allow that without printing an error.

We still want to reject the actual parsin since for that we need
the child device size to be at least 33 bytes. So we should still
check for that, but let's make it print a debug message only instead
of an error.

While at it, toss in a BUILD_BUG_ON() to verify our struct
old_child_dev_config is in fact 33 bytes.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1450110229-30450-8-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>