i915/gem_exec_balancer: Wait for both engines to complete before resubmitting

As the scratch buf is shared between the two requests on both engines,
we need to wait for both to finish using the buffer before we reset it.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>