i965/gen6: Reduce updates of transform feedback offsets with HW contexts.

i965/gen6: Reduce updates of transform feedback offsets with HW contexts.

The software-tracked transform feedback offsets (svbi_0_starting_index)
are incorrect in the presence of primitive restart, so we were actually
updating it with a bogus value if the batch wrapped and we emitted the
packet again during a single transform feedback.  By reducing state
emission, we avoid the bug.

Fixes piglit OpenGL 3.1/primitive-restart-xfb flush
Reviewed-by: Paul Berry <stereotype441@gmail.com>
NOTE: This is a candidate for the 9.1 branch.