drm/i915/guc: Remove stale comment for q_fail

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Fixes: 774439e1

 ("drm/i915/guc: re-optimise i915_guc_client layout")
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170518113104.54400-1-michal.wajdeczko@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>