drm/amdgpu/vce3: don't forget to tear down some rings

We can use .num_rings for that.

Fixes: 6f0359ff ("vce3: add support for third vce ring")
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>