drm/amd/powerplay: fix memory leak in smu7_hwmgr

add pp_smu7_thermal_fini function to free related
data when smu7_hwmgr_fini.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>