drm/i915/opregion: make VBT size limit more strict

The VBT in OpRegion should fit in mailbox #4.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/4bdb3f5820e3dbd1cdfa7b65cadfce4f80b880f0.1450089383.git.jani.nikula@intel.com