drm/ttm: Ignore signaled move fences

Move fences that have already signaled should not prevent memory
allocations with no_wait_gpu.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210227034524.21763-1-Felix.Kuehling@amd.com


Signed-off-by: Christian König <christian.koenig@amd.com>