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: NTvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: a091d4ee ("drm/i915: Hold a wakeref for probing the ring registers")
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: NMika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180212093928.6005-1-chris@chris-wilson.co.uk
(cherry picked from commit 74d00d28)
Signed-off-by: NRodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/i915/intel_engine_cs.c

@@ -1458,7 +1458,9 @@ 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) !=