[Why]
To support inbox1 in CW4 we need to actually program CW4 instead of
region 4 for newer firmware.

This is done correctly on DCN20/DCN21 but this code wasn't added to
DCN30.

[How]
Copy over the missing code. It doesn't need address translation since
DCN30 uses virtual addressing.
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>