drm/amdgpu: use a fence array for VMID management

Just wait for any fence to become available, instead
of waiting for the last entry of the LRU.

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>