IIRC apple used hardware to dictate what gpu to use, so without disabling the hardware there is no software that would be able to make the change for you. You would have to go into the bios and change a setting, that would then disable the discrete graphic chip. I would high recommended NOT downloading anything off the internet that says it would fix that problem. I unfortunately do not have a comparable machine laying around that I could test a fix, and give a step by step.

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Try booting into safe(single user mode) and deleting the drivers for the discrete chip.

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