drm/i915: Simple selftest to exercise live requests

Just create several batches of requests and expect it to not fall over!

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/20170213171558.20942-13-chris@chris-wilson.co.uk