Fix: GPv3: Editable channels filtering was inverted for grease pencil channels

As pointed out in #111426, the filtering for only editable channels
was wrongly set for grease pencil layers,
the layer being editable if it is not locked, and not the other way around.

Pull Request: https://projects.blender.org/blender/blender/pulls/111475

Co-authored-by: Pratik Borhade <PratikPB2123>

source/blender/editors/animation/anim_filter.cc

@@ -1776,7 +1776,7 @@ static size_t animdata_filter_grease_pencil_layer(ListBase *anim_data,
   }
 
   /* Only if the layer is editable. */
-  if ((filter_mode & ANIMFILTER_FOREDIT) && !layer.is_locked()) {
+  if ((filter_mode & ANIMFILTER_FOREDIT) && layer.is_locked()) {
     return items;
   }