if the src for a replace_buffer call is mapped after replacement:
* avoid clearing access flags
* update valid range
the pointer access here is always safe because the only case in which
this scenario can occur is if tc is forced to sync immediately after
creating a replaceent buffer, and the replacement buffer's lifetime
will always be exceeded by the lifetime of the real buffer
cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30107>
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