drm/atomic: clear out fence when duplicating state

[Fixed differently in 4.10]

The fence needs to be cleared out, otherwise the following commit
might wait on a stale fence from the previous commit. This was fixed
as a side effect of 96260142 (drm/fence: add in-fences support)
in kernel 4.10.

As this commit introduces new functionality and as such can not be
applied to stable, this patch is the minimal fix for the kernel 4.9
stable series.
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/drm_atomic_helper.c

@@ -3115,6 +3115,8 @@ void __drm_atomic_helper_plane_duplicate_state(struct drm_plane *plane,
 
 	if (state->fb)
 		drm_framebuffer_reference(state->fb);
+
+	state->fence = NULL;
 }
 EXPORT_SYMBOL(__drm_atomic_helper_plane_duplicate_state);