drm/atomic: clear out fence when duplicating state

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 9626014258a5 (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>