drm: zte: checking for NULL instead of IS_ERR()

drm_dev_alloc() never returns NULL, it only returns error pointers on
error.

Fixes: 0a886f59 ("drm: zte: add initial vou drm driver")
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161115095301.GC15424@mwanda

drivers/gpu/drm/zte/zx_drm_drv.c

@@ -107,8 +107,8 @@ static int zx_drm_bind(struct device *dev)
 		return -ENOMEM;
 
 	drm = drm_dev_alloc(&zx_drm_driver, dev);
-	if (!drm)
-		return -ENOMEM;
+	if (IS_ERR(drm))
+		return PTR_ERR(drm);
 
 	drm->dev_private = priv;
 	dev_set_drvdata(dev, drm);