drm/i915/bios: don't pass bdb to parsers that don't parse VBT directly

Hint that you're not supposed to look at VBT in these functions.

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/b82c326be8c796a70bdc2bd1c479bbb6159f5cb0.1506586821.git.jani.nikula@intel.com