[Why]
DCN20 and DCN21 have different phy programming sequences.

[How]
Create a separate dcn21_link_encoder for Renoir
Signed-off-by: NBhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: NRoman Li <Roman.Li@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>