drm/gma500: use NULL instead of using plain integer as pointer

This fixes the following sparse warnings:
drivers/gpu/drm/gma500/psb_drv.c:303:56: warning: Using plain integer as
NULL pointer

Reported-by: Abaci Robot <[e-mail]>
Signed-off-by: Yang Li <[e-mail]>
Signed-off-by: Patrik Jakobsson <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/gma500/psb_drv.c

···
 
 	ret = -ENOMEM;
 
-	dev_priv->mmu = psb_mmu_driver_init(dev, 1, 0, 0);
+	dev_priv->mmu = psb_mmu_driver_init(dev, 1, 0, NULL);
 	if (!dev_priv->mmu)
 		goto out_err;