drm/amd/powerplay: update how to use metrics table on Sienna Cichlid

Retrieve only those data interested instead of the whole table.

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>