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>