drm/i915/selftests: Make the mman object busy everywhere

Loop over all engines, issuing a request for the object on each in order
to make sure we leave no stone unturned when creating an active ref. The
purpose is to make sure that we can reap a zombie object (one that is
only alive due to an active reference on the GPU) no matter where that
active reference emanates from.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191022101704.5618-1-chris@chris-wilson.co.uk