drm/i915: Add missing kerneldoc for guc_client_alloc:engines

Brief parameter text to describe @engines.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dave Gordon <david.s.gordon@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471437762-22936-1-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

drivers/gpu/drm/i915/i915_guc_submission.c

@@ -744,6 +744,7 @@ static void guc_init_doorbell_hw(struct intel_guc *guc)
 /**
  * guc_client_alloc() - Allocate an i915_guc_client
  * @dev_priv:	driver private data structure
+ * @engines:	The set of engines to enable for this client
  * @priority:	four levels priority _CRITICAL, _HIGH, _NORMAL and _LOW
  * 		The kernel client to replace ExecList submission is created with
  * 		NORMAL priority. Priority of a client for scheduler can be HIGH,