drm/i915/selftests: Hold rpm for unparking

The call to i915_gem_unpark() checks that we hold a rpm wakeref before
taking a long term wakeref for i915->gt.awake. We should therefore make
sure we do hold the wakeref when directly calling unpark to disable
the retire worker.

Fixes: 932cac10 ("drm/i915/selftests: Prevent background reaping of active objects")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180809063449.4474-1-chris@chris-wilson.co.uk