drm/i915/selftests: Keep the engine awake while we keep for preemption

Keep the engine awake to ensure that we don't inject any pm-idle
requests.

References: https://bugs.freedesktop.org/show_bug.cgi?id=111108
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190912122639.25224-1-chris@chris-wilson.co.uk