drm/i915/selftests: Set igt_spinner.gt for early exit

Set up a default gt pointer for an early cleanup of igt_spinnter, before
a request is created and igt_spinner.gt set to the active engine's.

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/20190708215524.31639-1-chris@chris-wilson.co.uk