drm/i915: Track cursor changes as frontbuffer tracking flushes

We treat other plane updates in the same fashion. Spotted because
Rodrigo kept reporting a bug in the PSR code where the frontbuffer was
eternally stuck with a dirty cursor bit set.

The psr testcase should have caught this, but that i-g-t is kaputt.
Rodrigo is signed up to fix that.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Tested-by-and-Reviewed-by: NRodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -11782,6 +11782,10 @@ intel_cursor_plane_update(struct drm_plane *plane, struct drm_crtc *crtc,
 		return intel_crtc_cursor_set_obj(crtc, obj, crtc_w, crtc_h);
 	} else {
 		intel_crtc_update_cursor(crtc, visible);
+
+		intel_frontbuffer_flip(crtc->dev,
+				       INTEL_FRONTBUFFER_CURSOR(intel_crtc->pipe));
+
 		return 0;
 	}
 }