drm/tegra: dc: Using NULL instead of plain integer

Fixes the following sparse warnings:

drivers/gpu/drm/tegra/dc.c:2181:69: warning:
  Using plain integer as NULL pointer
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>

drivers/gpu/drm/tegra/dc.c

@@ -2160,7 +2160,7 @@ static int tegra_dc_couple(struct tegra_dc *dc)
 		struct device_link *link;
 		struct device *partner;
 
-		partner = driver_find_device(dc->dev->driver, NULL, 0,
+		partner = driver_find_device(dc->dev->driver, NULL, NULL,
 					     tegra_dc_match_by_pipe);
 		if (!partner)
 			return -EPROBE_DEFER;