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.
14 jobs for !734 with iris-preempt-tidy in 12 minutes and 24 seconds (queued for 7 seconds)
latest detached
Status Job ID Name Coverage
  Containers Build
passed #261782
debian

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  Build+Test
passed #261784
meson-clang

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passed #261790
meson-gallium-clover-llvm

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passed #261791
meson-gallium-clover-llvm39

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passed #261789
meson-gallium-drivers-other

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passed #261788
meson-gallium-radeonsi

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passed #261792
meson-gallium-st-other

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passed #261783
meson-gallium-swr

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passed #261787
meson-glvnd

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meson-loader-classic-dri

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passed #261785
meson-vulkan

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passed #261794
scons-llvm

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passed #261793
scons-nollvm

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passed #261795
scons-swr

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