Make sure main thread won't update last_sched fence when entity
is cleanup.

Fix a racing issue which is caused by putting last_sched fence
twice. Running vulkaninfo in tight loop can produce this issue
as seeing wild fence pointer.

v2: squash in build fix (Christian)
Signed-off-by: NPixel Ding <Pixel.Ding@amd.com>
Reviewed-by: NChristian König <christian.koenig@amd.com>
Reviewed-by: NMonk Liu <Monk.Liu@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>