drm/i915/gt: Empty uabi engines list during intel_engines_release()

While the uabi_engines_llist is populated in intel_engines_init() during
driver load, the corresponding function intel_engines_release() does not
correctly get rid of it. This can lead to a UAF if, after failed
initialization (for example when gt is set wedged on init), we try to
access the engines.

Suggested-by: Chris Wilson <chris.p.wilson@linux.intel.com>
Signed-off-by: Krzysztof Niemiec <krzysztof.niemiec@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240801154047.115176-2-krzysztof.niemiec@intel.com