drm/i915: Store preemption capability in engine->flags

Let's avoid having to delve down the pointer chain to see if the i915
device has support for preemption and store that on the engine, which
made the decision in the first place!

v2: Refactor common preemption policy between execlists/guc.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tomasz Lis <tomasz.lis@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180403183537.5522-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_lrc.c

@@ -183,7 +183,8 @@ static inline bool need_preempt(const struct intel_engine_cs *engine,
 				const struct i915_request *last,
 				int prio)
 {
-	return engine->i915->preempt_context && prio > max(rq_prio(last), 0);
+	return (intel_engine_has_preemption(engine) &&
+		__execlists_need_preempt(prio, rq_prio(last)));
 }
 
 /**
@@ -2117,11 +2118,13 @@ static void execlists_set_default_submission(struct intel_engine_cs *engine)
 	engine->unpark = NULL;
 
 	engine->flags |= I915_ENGINE_SUPPORTS_STATS;
+	if (engine->i915->preempt_context)
+		engine->flags |= I915_ENGINE_HAS_PREEMPTION;
 
 	engine->i915->caps.scheduler =
 		I915_SCHEDULER_CAP_ENABLED |
 		I915_SCHEDULER_CAP_PRIORITY;
-	if (engine->i915->preempt_context)
+	if (intel_engine_has_preemption(engine))
 		engine->i915->caps.scheduler |= I915_SCHEDULER_CAP_PREEMPTION;
 }