drm: fix syncobj null_fence_enable_signaling

That is certainly totally nonsense. dma_fence_enable_sw_signaling()
is the function who is calling this callback.

Signed-off-by: Chunming Zhou <david1.zhou@amd.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/246535/