drm/i915: Clarify type evolution of uabi_node/uabi_engines

Chaining user engines happens in multiple passes during driver
initialization, mutating its type along the way. It starts off with a
simple lock-less linked list (struct llist_node/head) populated by
intel_engine_add_user() which later gets sorted and converted to an
intermediate regular list (struct list_head) just to be converted once
more to its final rb-tree structure (struct rb_node/root) in
intel_engines_driver_register().

All of these types overlay the uabi_node/uabi_engines members which is
unfortunate but safe if one takes care about using the rb-tree based
structure only after the conversion has completed. However, mistakes
happen and commit 1ec23ed7 ("drm/i915: Use uabi engines for the
default engine map") violated that assumption, as the multiple type
evolution was all to easy hidden behind casts papering over it.

Make the type evolution of uabi_node/uabi_engines more visible by
putting all members into an anonymous union and use the correctly typed
member in its various users. This allows us to drop quite some ugly
casts and, hopefully, make the evolution of the members better
recognisable to avoid future mistakes.

Signed-off-by: Mathias Krause <minipli@grsecurity.net>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230928182019.10256-3-minipli@grsecurity.net