This should reflect what we actually support, while still being a superset of what native drivers actually support.
Native driver support varies, but, as the comments document, native drivers (at least as of windows 10) support only RGB -> RGB, YUV -> RGB, and BC -> RGB conversions. The actual formats supported are a more limited subset than exposed (despite that the video cards should be perfectly capable of supporting more conversions) but erring on the side of overreporting seems safer.
-- v2: wined3d: Implement wined3d_check_device_format_conversion().