drm/i915: Add a simple request selftest for waiting

A trivial kselftest to submit a request and wait upon it.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170213171558.20942-11-chris@chris-wilson.co.uk