drm/i915: clarify gen2 pageflip cmd

I've reviewed gen2 pageflip code to hunt down multiple prepare pageflip
issues. The only thing I've found is a slight but functionally
meaningless confusion about the length of the mi cmd.

Fix it up and add a comment about what this dword should be (according
to docs at least).
Reviewed-by: NEric Anholt <eric@anholt.net>
Signed-Off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -7288,7 +7288,7 @@ static int intel_gen2_queue_flip(struct drm_device *dev,
 		 MI_DISPLAY_FLIP_PLANE(intel_crtc->plane));
 	OUT_RING(fb->pitches[0]);
 	OUT_RING(obj->gtt_offset + offset);
-	OUT_RING(MI_NOOP);
+	OUT_RING(0); /* aux display base address, unused */
 	ADVANCE_LP_RING();
 out:
 	return ret;