drm/amd/display: Only wait for DMUB phy init on dcn21

[Why]
The wait for PHY init won't finish if the firmware doesn't support it.

[How]
Only hook this functionality up on DCN21 and move it out of DCN20.

For ASIC without support then this should return OK so we don't hang
while waiting in DC.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>