To make https://gitlab.winehq.org/wine/wine/-/merge_requests/5422 simpler. This will also make it easier to write the mode list to the registry as a single binary blob, although it is a larger change that could be done later.
-- v3: win32u: Add all display device source modes at once. win32u: Keep the primary current mode in the device manager context. win32u: Remove fake source creation when adding display mode. win32u: Close device manager source key in write_source_to_registry.