gallium/u_threaded_context: don't call memcpy in tc_set_constant_buffer

In sysprof, the CPU time spend in tc_set_constant_buffer drops
from 7% to 1.4% in a release build. It doesn't make sense to me.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6952>

src/gallium/auxiliary/util/u_threaded_context.c

@@ -696,7 +696,9 @@ tc_set_constant_buffer(struct pipe_context *_pipe,
       } else {
          tc_set_resource_reference(&p->cb.buffer,
                                    cb->buffer);
-         memcpy(&p->cb, cb, sizeof(*cb));
+         p->cb.user_buffer = NULL;
+         p->cb.buffer_offset = cb->buffer_offset;
+         p->cb.buffer_size = cb->buffer_size;
       }
    } else {
       memset(&p->cb, 0, sizeof(*cb));