drm/amd/pm: enable LCLK DS

Enable LCLK deep sleep and it works if we enable ASPM:
modprobe amdgpu aspm=1

Signed-off-by: Kenneth Feng <kenneth.feng@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>