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

the vega20 asic uses legacy powerplay driver by default.

1. cleanup is_support_sw_smu_xgmi() function.
(only use for vega20 xgmi pstate check)
2. by default, the vega20 set xgmi pstate by legacy powerplay routine.

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>