drm/amdgpu/powerplay: fix NULL pointer issue when SMU disabled

Fix smu related NULL pointer issue which occurs when SMU is disabled.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>