drm/amdgpu: fix the issue that apu has no smu firmware binary

The driver needn't load smu binary on APU platforms.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Madhav Chauhan <madhav.chauhan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>