Attach an external screen. If the same fault shows, it's a bad GPU (mainboard replacement time). If not, it's either a bad screen (more likely) or a bad wiring harness (less likely).
Kindly test with an external monitor as advised by ejn63 and keep us posted.
If the system is under warranty, we can get it serviced. Please click my DELL-username and write me a private conversation with the service tag and your contact details(Name, Phone#, Address and Email). If there is no warranty, then you could contact our team to get a quote for a paid service call - http://dell.to/1vnT6CQ
Let us know if you have any other queries.