It would be hard to argue that AMD has a monopoly, so therefore you are right - it is unlikely they'd be forced.
However, if NVIDIA really needs that tech, they can just start violating the patent. AMD sues. NVIDIA countersues AMD for violation of NVIDIA patents (it is almost guaranteed that MAD violates some of NVIDIA's patents). In the end, they either fight it out in court, or reach a settlement. Either way, the resolution of the conflict is that somebody determines the difference in value between the patent violations, and that difference gets paid. So either NVIDIA pays the difference to AMD, or vice versa. And afterwards, both parties have licenses of each other's patent portfolios. Kinda like what just happened between NVIDIA and Intel.
The only downside of this approach is that it might end up costing NVIDIA a pretty penny, but if they really think they need to use x86-64, this is one way to do so.