drm/amd/powerplay: update the data structure for NV12 SmuMetrics

Although it does not bring any problem for now, the coming gpu
metrics interface needs to handle them differently based on the
asic type.

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>