drm/i915: Don't wake the device up to check if the engine is asleep

If the entire device is powered off, we can safely assume that the
engine is also asleep (and idle).

Reported-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: a091d4ee931b ("drm/i915: Hold a wakeref for probing the ring registers")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180212093928.6005-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_engine_cs.c

@@ -1464,7 +1464,9 @@ static bool ring_is_idle(struct intel_engine_cs *engine)
 	struct drm_i915_private *dev_priv = engine->i915;
 	bool idle = true;
 
-	intel_runtime_pm_get(dev_priv);
+	/* If the whole device is asleep, the engine must be idle */
+	if (!intel_runtime_pm_get_if_in_use(dev_priv))
+		return true;
 
 	/* First check that no commands are left in the ring */
 	if ((I915_READ_HEAD(engine) & HEAD_ADDR) !=