iris: Fix zeroing of transform feedback offsets in strange cases.

iris: Fix zeroing of transform feedback offsets in strange cases.

Some of the dEQP.functional.transform_feedback tests end up doing
the following sequence of operations:

   1. BeginTransformFeedback
   2. PauseTransformFeedback
   3. Draw
   4. ResumeTransformFeedback

At step 1, we'd pack 3DSTATE_SO_BUFFER commands saying to zero the
SO_WRITE_OFFSET registers.  At step 2, we disable streamout, so step 3
doesn't bother emitting those commands.  Then, step 4 re-packs new
3DSTATE_SO_BUFFER commands with offset = 0xFFFFFFFF, saying to continue
appending at the existing offset.  This loads the value from the BO as
the offsets - but we never actually zeroed it.

So, just maintain a flag saying "we actually emitted the commands",
and stomp offset back to zero until we emit some.