i915/gem_exec_fence: Reduce non-preemptible workload

Non-preemptible spinners should be avoided as often as possible.
gem_exec_fence@[basic-await, nb-await] has no reason to use non-preemptible
workload, only hanging subtests need it.

Signed-off-by: Dominik Grzegorzek <dominik.grzegorzek@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>