drm/i915: Test simultaneously submitting requests to all engines

Use a recursive-batch to busy spin on each to ensure that each is being
run simultaneously.

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/20170213171558.20942-14-chris@chris-wilson.co.uk