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>