iris: Fix partial fast clear checks to account for miplevel.

Kenneth Graunke requested to merge kwg/mesa:iris-fastclear-mip into master

We enabled fast clears at level > 0, but didn't minify the dimensions when comparing the box size, so we always thought it was a partial clear and as a result never actually enabled any.

This eliminates some slow clears in Civilization VI, but they are mostly during initialization and not the main rendering.

Thanks to Dan Walsh for noticing we had too many slow clears.

Fixes: 393f659e ("iris: Enable fast clears on other miplevels and layers than 0.")


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