drm/exynos: change zero to NULL for sparse
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 <ff341aa343d564f9e53e9dcb6996be8c04859a66@oracle.com>
Signed-off-by: Inki Dae <bae4a7a27bbc965064e97edd6e179ac4979db440@samsung.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs