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)