This doesn't sound right though, and although I understand that this registry key may need to use the integer indices matching the .\DISPLAY indices, I think we'll then need to introduce some mapping in win32u to match these indices with the physical adapters reported by the drivers.
.\DISPLAY indices only correspond to the index the physical adapter was enumerated at, they do not match a specific physical adapter. Lets consider a scenario where a new adapter appears, such as when hot-plugging. Whether we sort the physical adapters or not on the driver side, and unless it ends up last, the mapping between .\DISPLAY indices and physical adapters will be different.
In win32u we use that registry key to save the display settings. The current and enumerated modes are refreshed on every enumeration, but the registry settings is kept from the last time it was written. When the .\DISPLAY to physical index mapping changes, we will incorrectly use the registry settings that was written for the old physical adapter, instead of creating one from scratch for that new adapter.
So in my opinion we need to have a physical adapter id -> logical index mapping in win32u, and keep the display settings linked to the physical adapters. This will also save a lot of complexity on the driver side as they won't need to sort anything anymore.