drm/amdgpu: cache the fence to wait for a VMID

Beneficial when a lot of processes are waiting for VMIDs.

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