drm/amdgpu: Use real power source in powerplay instand of hardcode

1. move ac_power to struct pm from dpm, so can be shared with powerplay
2. remove power_source in powerplay, use adev->pm.ac_power instand.
3. update ac_power before dispatch power task.

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>