drm/amdgpu: fix old fence check in amdgpu_fence_emit

commit 3d2aca8c upstream.

We don't hold a reference to the old fence, so it can go away
any time we are waiting for it to signal.

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>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>