iris: Zero the compute predicate when changing the render condition

1. Set a render condition.  We emit it immediately on the render
   engine, and stash q->bo as ice->state.compute_predicate in case
   the compute engine needs it.

2. Clear the render condition.  We were incorrectly leaving a stale
   compute_predicate kicking around...

3. Dispatch compute.  We would then read the stale compute predicate,
   and try to load it into MI_PREDICATE_DATA.  But q->bo may have been
   freed altogether, causing us to try and use garbage memory as a BO,
   adding it to the validation list, failing asserts, and tripping
   EINVALs in execbuf.

Huge thanks to Mark Janes for narrowing this sporadic GL CTS failure
down to a list of 48 tests I could easily run to reproduce it.  Huge
thanks to the Valgrind authors for the memcheck tool that immediately
pinpointed the problem.

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