drm/i915/selftests: Check preemption support on each engine

Check that we have setup on preemption for the engine before testing,
instead warn if it is not enabled on supported HW.

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/20190306142517.22558-28-chris@chris-wilson.co.uk