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 <[email protected]>
Reviewed-by: Oleksandr Andrushchenko <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

Author: Ding Xiang

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;
 	}