drm/amdgpu: add pm metrics structure definition

Define the pm metrics structures to be exposed via sysfs.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Yang Wang <kevinyang.wang@amd.com>