i965: Only flush the batchbuffer if we need to zero the SO offsets

i965: Only flush the batchbuffer if we need to zero the SO offsets

If we don't have pipelined register access (e.g. Haswell before kernel
v4.2), then we can only implement EXT_transform_feedback by reseting the
SO offsets *between* batches. However, if we do have pipelined access to
the SO registers on gen7, we can simply emit an inline reset of the SO
registers without a full batch flush.

v2 [by Ken]: Simplify after recent kernel feature detection changes.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>