drm/i915/selftests: Smoketest preemption

Very light stress test to bombard the submission backends with a large
stream with requests of randomly assigned priorities. Preemption will be
occasionally requested, but unlikely to ever succeed! (Although we may
build a long queue of requests and so may trigger an attempt to inject a
preempt context, as we emit no batch, the arbitration window is limited
to between requests inside the ringbuffer. The likelihood of actually
causing a preemption event is therefore very small. A later variant
should try to improve the likelihood of preemption events!)

v2: Include a second pattern with more frequent preemption

Signed-off-by: Chris Wilson <[email protected]>
Cc: Michał Winiarski <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Reviewed-by: Tvrtko Ursulin <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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