vc4: Don't try to put our dmabuf-exported BOs into the BO cache.

We'd sometimes try to reallocate something that X was using as a new
pipe_resource, and potentially conflict in our rendering.  But even
worse, if we reallocated the BO as a shader, the kernel would reject
rendering using the shader.

src/gallium/drivers/vc4/vc4_bufmgr.c

@@ -286,6 +286,7 @@ vc4_bo_get_dmabuf(struct vc4_bo *bo)
                         bo->handle);
                 return -1;
         }
+        bo->private = false;
 
         return fd;
 }