intel/blorp: Disable sampler state prefetching on Gen11

intel/blorp: Disable sampler state prefetching on Gen11

Sampler state prefetching is broken on Gen11, and WA_160668216 says
to disable it.  Apparently sampler state prefetching also has basically
zero impact on performance, so we don't need to worry there.

i965, anv, and iris already handle this correctly, but we missed BLORP.
Ideally the kernel should globally disable this by writing SARCHKMD, at
which point we wouldn't have to worry about it.  But let's be defensive
and handle it ourselves too.

v2: separate out from BTP workaround in case we change that eventually

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com> [v1]