Shows all Cycles panels based on scene, not context engine

This is tricky since we may want granular polling depending on the setting.
Or an option to pick whether we want the context or the scene to drive the
panels to prevent too many panels when mixing Eevee and Cycles for example.

intern/cycles/blender/addon/ui.py

@@ -50,7 +50,7 @@ class CyclesButtonsPanel:
 
     @classmethod
     def poll(cls, context):
-        return context.engine in cls.COMPAT_ENGINES
+        return context.scene.view_render.engine in cls.COMPAT_ENGINES
 
 
 def get_device_type(context):