drm/amd/powerplay: check whether SMU IP is enabled before access

Since on early phase of bringup, the SMU IP may be not enabled or
supported. Without this, we may hit null pointer dereference on
accessing smu->adev.
Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Yong Zhao <Yong.Zhao@amd.com>
Tested-by: Yong Zhao <Yong.Zhao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>