drm/amd/powerplay: simplify the interface of get_gpu_power

this callback function is only call in read_sensor in smu_v11_0.c,
so move this code to {asic}_ppt.c to implement as asic related function.

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>