iris: Enable HiZ for non-8x4 aligned miplevels on Icelake and later
8x4 alignment was absolutely required prior to Gfx8, and while some things were relaxed on Gfx8-9, Nanley's experiments in issue #3788 (closed) indicated that there were still issues on those platforms.
It appears that the restrictions were relaxed on Icelake and non-8x4 aligned HiZ "should" work on Gfx11+.
2022: Now rebased and rewritten for intervening changes, retargeted at Gfx11+ only.
Car Chase results on my Tigerlake NUC are 0.0945704% +/- 0.0726234% (n=20). INTEL_MEASURE=1 shows that there are no more hiz ambiguates. However, the slow clear mentioned in #5258 (closed) still persists due to another restriction not fixed in this series.