radeonsi: set bo_size for user memory allocations

Otherwise the assert in si_set_shader_buffer could trigger for blits
through clear_buffer on user resources.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29867>

src/gallium/drivers/radeonsi/si_buffer.c

@@ -641,6 +641,7 @@ static struct pipe_resource *si_buffer_from_user_memory(struct pipe_screen *scre
    }
 
    buf->gpu_address = ws->buffer_get_virtual_address(buf->buf);
+   buf->bo_size = templ->width0;
    return &buf->b.b;
 }