You should be able to change the primary display in the Nvidia Control Panel.
All right, I don't know if this would help but...
While your pc is on, disconnect the secondary display.
Reboot, and let the displayport monitor become the primary display.
Then connect the secondary display.
Reboot and see if the displayport monitor stays as the primary display.
If this doesn't work, would you have a displayport to DVI adapter cable to try?
Someone said unchecking fast startup solved this problem for them.