In preparation for nulldrv display modes support.
-- v7: win32u: Move display placement logic out of graphics drivers. win32u: Move full display mode lookup out of graphics drivers. win32u: Use current mode position if desired mode doesn't specify it. win32u: Always copy devmode in validate_display_settings. win32u: Read registry or current mode when validation needs it. win32u: Do not keep display modes driver extra in the registry.