iGPU is usually auto disabled when discrete card (GTX770) is plugged in mobo's PCIE slot.
You can select default primary display in the BIOS from CPU Graphics or PCIe graphical devices.
I am having the same issue, and can't find where in the BIOS I disable this option.
you can revert it to default by reset CMOS, or refer to your motherboard manual for the option. by default it should use the igpu when you take out the discrete card.