drm/amdgpu: sw pstate switch should only be for vega20

Driver steered p-state switching is designed for Vega20 only.
Also simplify early return for temporary disable due to SMU FW
bug.

Signed-off-by: Jonathan Kim <Jonathan.Kim@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>