drm/i915: fix VBT parsing for SDVO child device mapping

commit 75067dde
Author: Antti Koskipaa <antti.koskipaa@linux.intel.com>
Date:   Fri Jul 10 14:10:55 2015 +0300

    drm/i915: Per-DDI I_boost override

increased size of union child_device_config without taking into account
the size check in parse_sdvo_device_mapping(). Switch the function over
to using the legacy struct only.

Fixes: 75067dde ("drm/i915: Per-DDI I_boost override")
Cc: Antti Koskipaa <antti.koskipaa@linux.intel.com>
Cc: David Weinehall <david.weinehall@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>