drm/i915/opregion: try to validate RVDA VBT only if it's there

Seems more sensible this way, and reduces indent for the more common
case.

Reviewed-by: Bob Paauwe <bob.j.paauwe@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1490783578-6065-2-git-send-email-jani.nikula@intel.com