drm/amd/powerplay: change backend allocation to backend init

backend_init and backend_fini are paired functions, backend is freed
in backend_fini and should be allocated in backend_init.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Eric Huang <JinHuiEric.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>