drm/xen: fix passing zero to 'PTR_ERR' warning

Fix a static code checker warning:
    drivers/gpu/drm/xen/xen_drm_front.c:404 xen_drm_drv_dumb_create()
    warn: passing zero to 'PTR_ERR'
Signed-off-by: Ding Xiang <dingxiang@cmss.chinamobile.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1585562347-30214-1-git-send-email-dingxiang@cmss.chinamobile.com

drivers/gpu/drm/xen/xen_drm_front.c

@@ -401,7 +401,7 @@ static int xen_drm_drv_dumb_create(struct drm_file *filp,
 
 	obj = xen_drm_front_gem_create(dev, args->size);
 	if (IS_ERR_OR_NULL(obj)) {
-		ret = PTR_ERR(obj);
+		ret = PTR_ERR_OR_ZERO(obj);
 		goto fail;
 	}