drm/amd/powerplay: fix raven issue for sw smu

Raven series doesn't support sw smu so far, so we have to add checking to
confirm it return false while the asic is raven.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Likun Gao <Likun.Gao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>