Fix #130799: Prevent cycles material template to show in Grease Pencil

The `poll()` function for `CYCLES_PT_context_material` was using the
legacy `GPENCIL` identifier as opposed to `GREASEPENCIL`. This caused
duplicated material templates to show in the material tab.

Pull Request: https://projects.blender.org/blender/blender/pulls/130962

intern/cycles/blender/addon/ui.py

@@ -1156,7 +1156,7 @@ class CYCLES_PT_context_material(CyclesButtonsPanel, Panel):
 
     @classmethod
     def poll(cls, context):
-        if context.active_object and context.active_object.type == 'GPENCIL':
+        if context.active_object and context.active_object.type == 'GREASEPENCIL':
             return False
         else:
             return (context.material or context.object) and CyclesButtonsPanel.poll(context)