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 <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1615969652-80225-1-git-send-email-yang.lee@linux.alibaba.com

drivers/gpu/drm/gma500/psb_drv.c

@@ -333,7 +333,7 @@ static int psb_driver_load(struct drm_device *dev, unsigned long flags)
 
 	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;