drm/amd/powerplay: enable gpu_metrics export on legacy powerplay routines

Enable gpu_metrics support on legacy powerplay routines.

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