First, connect the MBP to an external monitor to check if the problem is caused by the GPU or by the screen. If the external monitor show a normal screen then the logic board should be OK. Knowing the GPU is OK now concentrate your attention to the internal display LVDS cable connection: lift up the top case and remove the LVDS cable connector from its socket then fully reinsert it. Put back the top case without screwing it, connect the AC adapter and boot the machine. If the video is normal then the repair is done. If not, the cable or the LCD panel could be faulty. You'll have to swap parts to solve the issue.