drm/amd/powerplay: add interface to get current shallow sleep clocks

This patch adds interface to get current shallow sleep clocks for display and in
smu v11 didn't have this implementation.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Kevin Wang <Kevin1.Wang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>