struct

Selects the DDI that the transcoder will connect to.

This field has a non-trivial value encoding. The ddi_*() and set_ddi_*()

helpers should be preferred to accessing the field directly.

This field is tagged `_subtle` because the definition matches the bits used

on Tiger Lake, but it's used on all supported models. Kaby Lake and Skylake

have a very similar field, which only takes up bits 30-28. Fortunately, bit

27 is reserved MBZ (must be zero). So, there's still a 1:1 mapping between

DDI selection and the values of bits 30-27.

We take advantage of this to avoid forking the entire (fairly large)

register definition by papering over this difference in the helpers

`ddi_kaby_lake()` and `set_ddi_kaby_lake()`.