drm/i915: Live testing of empty requests

Primarily to emphasize the difference between just advancing the
breadcrumb using a bare request and the overhead of dispatching an
execbuffer.

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/20170213171558.20942-16-chris@chris-wilson.co.uk