drm/i915/gt: Cleanup partial engine discovery failures

In the Linux kernel, the following vulnerability has been resolved: drm/i915/gt: Cleanup partial engine discovery failures If we abort driver initialisation in the middle of gt/engine discovery, some engines will be fully setup and some not. Those incompletely setup engines only have 'engine->release == NULL' and so will leak any of the common objects allocated. v2: - Drop the destroy_pinned_context() helper for now. It's not really worth it with just a single callsite at the moment. (Janusz)