drm/amdgpu: remove amdgpu_fence_is_signaled

It's just overhead to check the fence value
when we signal them directly anyway.

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