drm/i915: Add missing parameter to intel_dp_set_drrs_state documentation.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1472116022-17598-2-git-send-email-maarten.lankhorst@linux.intel.comReviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/intel_dp.c

@@ -5049,6 +5049,7 @@ static void intel_dp_pps_init(struct drm_device *dev,
 /**
  * intel_dp_set_drrs_state - program registers for RR switch to take effect
  * @dev: DRM device
+ * @crtc_state: a pointer to the active intel_crtc_state
  * @refresh_rate: RR to be programmed
  *
  * This function gets called when refresh rate (RR) has to be changed from