drm/i915/selftests: Always free spinner on __sseu_prepare error

Prepare a nice little onion unwind to ensure that we always free the
spinner if we __sseu_prepare fails.

Fixes: c06ee6ff ("drm/i915/selftests: Context SSEU reconfiguration tests")
Reported-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190215195010.16637-1-chris@chris-wilson.co.uk


Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>