i965/gen7: Enable transform feedback as long as kernel support is present.

i965/gen7: Enable transform feedback as long as kernel support is present.

The last major issue (intervening-read) is fixed, so let's turn this
on for real.  The only other known issue is a hardware limitation for
tesselation with flat shading.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>