drm/i915/selftests: Remove erroneous intel_engine_pm_put

On an allocation error path for live_hwsp_alternate, we dropped the
engine wakeref before we had even acquired it.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200209131922.180287-4-chris@chris-wilson.co.uk