iris: Lessen texture cache hack flush for blits/copies on Icelake.

iris: Lessen texture cache hack flush for blits/copies on Icelake.

Lionel found actual documentation for this at long last.  Apparently
it actually is a sampler cache limitation that was mostly fixed on
Icelake.  Unfortunately, it seems there are still issues with ASTC
and non-ASTC sampler views.  Still, we can lessen the flush condition
from "format mismatch" to "ASTC mismatch", which eliminates most of
the flushing here.

We also update the documentation to refer to the workaround name.