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 <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com> #v2
Link: https://patchwork.freedesktop.org/patch/msgid/20190806124300.24945-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_irq.c

@@ -3251,7 +3251,7 @@ gen11_engine_irq_handler(struct intel_gt *gt, const u8 class,
 	struct intel_engine_cs *engine;
 
 	if (instance <= MAX_ENGINE_INSTANCE)
-		engine = gt->i915->engine_class[class][instance];
+		engine = gt->engine_class[class][instance];
 	else
 		engine = NULL;