drm/amd/display: Fix CW4 programming for dmub30 cached inbox

[Why]
The conditions for whether we used cached vs non-cached inbox1 depend
on a version check that mismatches what the shared helpers in dmub20
implement.

[How]
Use the dmub_dcn20_use_cached_inbox check for dmub_dcn30 as well.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>