IIRC the VRMs all feed the same suply rail (Vcore). There is no 'dedicated' VRM or voltage rail for the iGPU, which is also why motherboards can have any number of power phases feeding the same CPU. More phases in theory improves stability, but in most use cases as long as the minimum number of phases to meet ripple spec are present (fewer than you'd think) and those phases are rated to handle the load, then everything is fine.