iris: Fix headerless sampler messages in compute shaders with preemption

We were failing to set the "Headerless Message for Preemptable Contexts" bit in SAMPLER_MODE in the compute context. Other drivers use a single hardware context, so setting it on the render engine was sufficient to flip it in both pipelines. But iris uses a separate hardware context for compute, so we were only getting these set for the render context.

Thanks to @jekstrand for catching this bug.