drm/amdgpu: cleanup SPM support a bit

This should probably not access job->vm and also emit the SPM switch
under the conditional execute.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>