drm/i915/selftests: Exercise concurrent submission to all engines

The simplest and most maximal submission we can do, a thread to submit
requests unto each engine.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190925193446.26007-1-chris@chris-wilson.co.uk