drm/amdgpu: Get real power source to initizlize ac_power

driver need to know the real power source to do some power
related configuration when initialize.

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