drm/amdgpu: get number of shade engine by cgs interface.

the num of shade engine was needed to
measure the activity of the graphics core
and to enable di/dt feature.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>