drm/i915: Fix typo in semaphore debug message

Pronounces less rude when fixed.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170220104657.5237-1-tvrtko.ursulin@linux.intel.com