drm/vgem: Fix return value check in vgem_init()

In case of error, the function platform_device_register_simple() returns
ERR_PTR() and never returns NULL. The NULL test in the return value
check should be replaced with IS_ERR().

Fixes: af33a9190d02 ("drm/vgem: Enable dmabuf import interfaces")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Fix fixes: tag per Chris' review.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170521011939.8111-1-weiyj.lk@gmail.com

drivers/gpu/drm/vgem/vgem_drv.c

@@ -438,8 +438,8 @@ static int __init vgem_init(void)
 
 	vgem_device->platform =
 		platform_device_register_simple("vgem", -1, NULL, 0);
-	if (!vgem_device->platform) {
-		ret = -ENODEV;
+	if (IS_ERR(vgem_device->platform)) {
+		ret = PTR_ERR(vgem_device->platform);
 		goto out_fini;
 	}