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/

drivers/gpu/drm/drm_syncobj.c

@@ -184,7 +184,6 @@ static const char *drm_syncobj_null_fence_get_name(struct dma_fence *fence)
 
 static bool drm_syncobj_null_fence_enable_signaling(struct dma_fence *fence)
 {
-    dma_fence_enable_sw_signaling(fence);
     return !dma_fence_is_signaled(fence);
 }