Try connecting an external monitor and check whether its' display is also flickering.
If if doesn't, then it might not be a problem with the GPU but possibly could be either:
1. faulty video cable between motherboard and screen (press around the screen 'hinges' area (this is where the cable passes through from the body to the lid) and see if you can make it better or worse.
2. faulty video cable connection either on the motherboard or on the screen.
3. faulty backlight power supply feed or lid sensor when the lid is closed or opened a signal is sent to the relevant section on the motherboard which sends a signal to the screen to software turn on / turn off the backlighting.
4. faulty LCD screen
If it does, then there is a problem with the GPU on the motherboard.
Here is a link to a video showing the dis-assembly of your laptop, which may be of some help.