drm/amdgpu: update module paramter doc of amdgpu_dpm

the vega20 isn't supported 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>