iris: Fix headerless sampler messages in compute shaders with preemption

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 Jason Ekstrand for catching this bug.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6380>