drm/panfrost: Only put sync_out if non-NULL

drm/panfrost: Only put sync_out if non-NULL

Dan Carpenter's static analysis tool reported:

drivers/gpu/drm/panfrost/panfrost_drv.c:222 panfrost_ioctl_submit()
error: we previously assumed 'sync_out' could be null (see line 216)

Indeed, sync_out could be NULL if userspace doesn't send a sync object
ID for the out fence.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lists.freedesktop.org/archives/dri-devel/2019-May/217014.html
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190509082151.8823-1-tomeu.vizoso@collabora.com