Cleanup: Use const when retrieving custom data layers

Knowing when layers are retrieved for write access will be essential
when adding proper copy-on-write support. This commit makes that
clearer by adding `const` where the retrieved data is not modified.

Ref T95842

source/blender/blenkernel/intern/multires.c

@@ -1021,7 +1021,7 @@ void multires_modifier_update_mdisps(struct DerivedMesh *dm, Scene *scene)
   CCGDerivedMesh *ccgdm = (CCGDerivedMesh *)dm;
   Object *ob;
   Mesh *me;
-  MDisps *mdisps;
+  const MDisps *mdisps;
   MultiresModifierData *mmd;
 
   ob = ccgdm->multires.ob;