drm/i915/gt: Move the [class][inst] lookup for engines onto the GT

drm/i915/gt: Move the [class][inst] lookup for engines onto the GT

To maintain a fast lookup from a GT centric irq handler, we want the
engine lookup tables on the intel_gt. To avoid having multiple copies of
the same multi-dimension lookup table, move the generic user engine
lookup into an rbtree (for fast and flexible indexing).

v2: Split uabi_instance cf uabi_class
v3: Set uabi_class/uabi_instance after collating all engines to provide a
stable uabi across parallel unordered construction.

Signed-off-by: Chris Wilson <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: Daniele Ceraolo Spurio <[email protected]>
Reviewed-by: Daniele Ceraolo Spurio <[email protected]> #v2
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]