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: NGrazvydas Ignotas <notasas@gmail.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>