drm/amd/powerplay: remove the support of vega20 from swsmu

by default, vega20 will use legacy powerplay driver.
in order to maintain the code conveniently in the future,
remove the support of vega20 from swsmu.

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>