i965/vs: Switch to the new VS backend by default.

i965/vs: Switch to the new VS backend by default.

Now instead of env INTEL_NEW_VS=1 to get it, you need INTEL_OLD_VS=1
to not get it.  While it's not quite to the same codegen efficiency as
the old backend, it is not regressing piglit on G965 and G45, and
actually fixing bugs on gen6, and the remaining codegen quality
regressions all appear tractable.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>