drm/amdgpu: fix pm_load_smu_firmware for SR-IOV

For SR-IOV VF, powerplay may not be supported, in this case,
error '-EINVAL' should not be returned.

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