drm/i915: Test request ordering between engines

A request on one engine with a dependency on a request on another engine
must wait for completion of the first request before starting.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170213171558.20942-15-chris@chris-wilson.co.uk