drm/amd/display: create dcn21_link_encoder files

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

[How]
Create a separate dcn21_link_encoder for Renoir

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>