drm/exynos: change zero to NULL for sparse

We recently changed this function to return a pointer instead of an int
so we need to change this zero to a NULL or Sparse complains:

	drivers/gpu/drm/exynos/exynos_drm_drv.h:346:47:
	warning: Using plain integer as NULL pointer
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>

drivers/gpu/drm/exynos/exynos_drm_drv.h

@@ -343,7 +343,7 @@ struct exynos_drm_display * exynos_dpi_probe(struct device *dev);
 int exynos_dpi_remove(struct device *dev);
 #else
 static inline struct exynos_drm_display *
-exynos_dpi_probe(struct device *dev) { return 0; }
+exynos_dpi_probe(struct device *dev) { return NULL; }
 static inline int exynos_dpi_remove(struct device *dev) { return 0; }
 #endif