zink: fix uncached memory readback

Mike Blumenkrantz requested to merge zmike/mesa:zink-cached into main

the inner conditional here didn't include uncached readback, meaning that many (most?) buffers allocated with uncached memory (i.e., BAR) were being read back directly instead of using staging resources to be faster

at some point this inner conditional should be reevaluated to determine whether it still does anything, but this is not that time

fixes, among other things, loading in DOOM2016 on some GPUs

Fixes: 52f27cda ("zink: allow direct memory mapping for any COHERENT+CACHED buffer")

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