freedreno: Flush batches upon destroying the ctx.

freedreno: Flush batches upon destroying the ctx.

The invalidate would take it out of the bc tracking, so you could go
allocate a new batch->idx matching this one, while this one is still in
the bc using that idx.

You can't generate any new rendering with the ctx's old batches at this
point, anyway, so just flush for simplicity.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11439>