drm/i915/selftests: Make the hanging request non-preemptible

In some of our hangtests, we try to reset an active engine while it is
spinning inside the recursive spinner. However, we also try to flood the
engine with requests that preempt the hang, and so should disable the
preemption to be sure that we reset the right request.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200607222108.14401-2-chris@chris-wilson.co.uk