drm/i915: Move over to intel_context_lookup()

In preparation for an ever growing number of engines and so ever
increasing static array of HW contexts within the GEM context, move the
array over to an rbtree, allocated upon first use.

Unfortunately, this imposes an rbtree lookup at a few frequent callsites,
but we should be able to mitigate those by moving over to using the HW
context as our primary type and so only incur the lookup on the boundary
with the user GEM context and engines.

v2: Check for no HW context in guc_stage_desc_init

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190308132522.21573-4-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -4655,15 +4655,20 @@ static int __intel_engines_record_defaults(struct drm_i915_private *i915)
 	}
 
 	for_each_engine(engine, i915, id) {
+		struct intel_context *ce;
 		struct i915_vma *state;
 		void *vaddr;
 
-		GEM_BUG_ON(to_intel_context(ctx, engine)->pin_count);
+		ce = intel_context_lookup(ctx, engine);
+		if (!ce)
+			continue;
 
-		state = to_intel_context(ctx, engine)->state;
+		state = ce->state;
 		if (!state)
 			continue;
 
+		GEM_BUG_ON(ce->pin_count);
+
 		/*
 		 * As we will hold a reference to the logical state, it will
 		 * not be torn down with the context, and importantly the