drm/i915: vbt defs typo fixes

No more sing-a-ling.

Reported-by: Adam Jackson <ajax@redhat.com>
You're-my-ding-a-ling-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: David Weinehall <david.weinehall@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180118150613.26140-1-jani.nikula@intel.com