anv,isl: Lower storage image formats in anv

anv,isl: Lower storage image formats in anv

ISL was being a bit too clever for its own good and lowering the format for
us.  This is all well and good *if* we always want to lower it.  However,
the GL driver selectively lowers the format depending on whether the
surface is write-only or not.

Reviewed-by: Chad Versace <chad.versace@intel.com>
Cc: "12.0" <mesa-stable@lists.freedesktop.org>