drm/xen-front: checking for NULL instead of IS_ERR

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

Fixes: c575b7eeb8 ("drm/xen-front: Add support for Xen PV display frontend")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180508092650.GA661@mwanda

drivers/gpu/drm/xen/xen_drm_front.c

@@ -543,8 +543,8 @@ static int xen_drm_drv_init(struct xen_drm_front_info *front_info)
 	front_info->drm_info = drm_info;
 
 	drm_dev = drm_dev_alloc(&xen_drm_driver, dev);
-	if (!drm_dev) {
-		ret = -ENOMEM;
+	if (IS_ERR(drm_dev)) {
+		ret = PTR_ERR(drm_dev);
 		goto fail;
 	}