drm/amd/powerplay: correct the value retrieved through GPU_LOAD sensor interface

the unit of variable AverageGfxActivity defined in smu12 metrics
struct is centi, so the retrieved value should be divided by 100 before
return.

Signed-off-by: Xiaomeng Hou <Xiaomeng.Hou@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>