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)

Signed-off-by: Chris Wilson <chris.p.wilson@intel.com>
Cc: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220915232654.3283095-2-matthew.d.roper@intel.com