Change `initial_device_display_name` param special case?

This issue has been migrated from [#4638](https://github.com/matrix-org/synapse/issues/4638).

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If we can bear to swap out a temporary special case for a permanent one, it would be good to override `initial_device_display_name` from the initial request params with the one from the current request instead of ignoring it in https://github.com/matrix-org/synapse/blob/master/synapse/rest/client/v2_alpha/register.py#L283

This would let us set device display names correctly when registering.

This would probably be a thing that goes in the spec too.