i965: Move GS_THREAD_END mlen calculations out of the generator.

i965: Move GS_THREAD_END mlen calculations out of the generator.

The visitor was setting a mlen that was wrong for Broadwell, but the
generator was ignoring it and doing the right thing regardless.  We may
as well move the logic fully into the visitor.  This will be useful in
the next commit as well.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>