drm/amdgpu: overwrite signaled fence in amdgpu_sync

This allows using amdgpu_sync even without peeking into the fences for a
long time.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>