drm/amdgpu: cleanup amdgpu_ring_fini

cleanup amdgpu_ring_fini to check the prerequisites before changing ring->sched.ready

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>