drm/i915/selftests: Include arbitration points in preemption smoketest

Include a batch full of a page of arbitration points in order to provide
a window for inject_preempt_context() in the preemption smoketests.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181001123204.23982-2-chris@chris-wilson.co.uk