This one is not too straight-forward as your BIOS has no "Disable" option for integrated graphics in the BIOS menu.
As a result, per 3-10 in the motherboard's manual, under the "Advanced" option of your BIOS, and the "Integrated Graphics Configuration" sub-menu, choose "PEG" (PCI-Express Graphics Device) under "Initiate Graphic Adapter [PEG]".
This is the closest thing I can imagine to setting the PCIe as the full-time graphics card in this BIOS.
I have to disable integrated graphics on my workstation laptop at work or I keep getting driver failed messages from the Nvidia driver software; though I have a clear "disable" option in the BIOS.