panvk: don't dirty preloaded tiles for writeback
Tiles that are only touched by the preload shader don't need to be
written back, because their contents will be the same. The comment
suggests that this was the original intent, and that the value of
clean_fragment_write
was just swapped by accident.
I ran dEQP-VK.renderpass.*
(which should exercise preload to some extent) and didn't see any regressions, but didn't do any performance measurement.