I don't see any point of mentioning DRM at all. Either I can take full control of the device and run whatever code I want, in which case its "Open Hardware", or I can't. In which case it's short of a fucking brick.
Of course, DRM-schemes do hamper adoption of free software, but only in respect of people not being able to play/process the content they want, because said schemes are probably not implemented in free software (out of fear the scheme might be defeated -- which of course is stupid in the first place; a working DRM is a mathematical impossibility). But DRM belongs to the "Open Content" and "Open File Formats" debate, and has nothing whatsoever to do with hardware in the first place.