drm/i915/execlists: Record preemption for selftests

Put back the preemption counters lost in commit 22b7a426
("drm/i915/execlists: Preempt-to-busy") so that our selftests that
assert no preemption took place continue to function.

v2: But a timeslice is only a "soft" preemption!

Fixes: 22b7a426

 ("drm/i915/execlists: Preempt-to-busy")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190710064454.682-1-chris@chris-wilson.co.uk