iris: Some tidying for preemption support
Just enable it during init_render_context on Gen10+, and move the Gen9 state tracking into iris_genx_state so it only exists on Gen9.
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The migration is almost done, at least the rest should happen in the background. There are still a few technical difference between the old cluster and the new ones, and they are summarized in this issue. Please pay attention to the TL:DR at the end of the comment.
Just enable it during init_render_context on Gen10+, and move the Gen9 state tracking into iris_genx_state so it only exists on Gen9.