drm/amdgpu: drop VMID per ring tracking

David suggested this a long time ago, instead of checking
each ring just walk over all the VMIDs in reverse LRU order.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Andres Rodriguez <andresx7@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>