drm/i915: Simplify check for idleness in hangcheck

Having fixed the tracking of the engine's last_submitted_seqno, we can
now rely on it for detecting when the engine is idle (and not have to
touch the requests pointer).

Testcase: igt/gem_exec_whisper
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460010558-10705-9-git-send-email-chris@chris-wilson.co.ukReviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>

drivers/gpu/drm/i915/i915_irq.c

@@ -2805,9 +2805,8 @@ static void gen8_disable_vblank(struct drm_device *dev, unsigned int pipe)
 static bool
 ring_idle(struct intel_engine_cs *engine, u32 seqno)
 {
-	return (list_empty(&engine->request_list) ||
-		i915_seqno_passed(seqno,
-				  READ_ONCE(engine->last_submitted_seqno)));
+	return i915_seqno_passed(seqno,
+				 READ_ONCE(engine->last_submitted_seqno));
 }
 
 static bool