drm/i915: refactor ddi translations into a separate file (v2)

Ville suggested this, these tables are probably better being
standalone.

This fixes up the cnl/bxt interfaces to be like the others,
the intel one I left alone since it has a few extra entrypoints.

v2: add back missing rocketlake bits.
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
[Jani: made some functions static]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/def9eed2581d71863ccdf35f323b525facc2482c.1612467466.git.jani.nikula@intel.com