[why]
If DCN30 watermark calc is used for DCN301, the calculated values are
wrong due to the data structure mismatch between DCN30 and DCN301.
However, using the original DCN301 watermark values causes underflow.

[how]
- Add DCN21-style watermark calculations
- Adjust DCN301 watermark values to remove the underflow
Reviewed-by: Zhan Liu <zhan.liu@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>