iris: Add iris_resolve_conditional_render().

iris: Add iris_resolve_conditional_render().

This function can be used to stall on the CPU and resolve the predicate
for the conditional render. It will convert ice->state.predicate from
IRIS_PREDICATE_STATE_USE_BIT to either IRIS_PREDICATE_STATE_RENDER or
IRIS_PREDICATE_STATE_DONT_RENDER, depending on the result of the query.

v2:
 - return void (Ken)
 - update the stored condition (Ken)
 - simplify the code leading to resolve the predicate (Ken)

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>