drm/amdgpu: fix a memory leak in fence cleanup when unloading

Commit b61badd2 ("drm/amdgpu: fix usage slab after free")
reordered when amdgpu_fence_driver_sw_fini() was called after
that patch, amdgpu_fence_driver_sw_fini() effectively became
a no-op as the sched entities we never freed because the
ring pointers were already set to NULL.  Remove the NULL
setting.

Reported-by: Lin.Cao <lincao12@amd.com>
Cc: Vitaly Prosyak <vitaly.prosyak@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Fixes: b61badd2 ("drm/amdgpu: fix usage slab after free")
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>