Cleanup: split BKE_scene_get_depsgraph() into two functions

Split the depsgraph allocation into a separate function
`BKE_scene_ensure_depsgraph()`. Parameters are only passed to those
functions that actually need them. This removes the the "if that boolean
is `false` this pointer is allowed to be `NULL`" logic and more cleanly
decouples code.

No functional changes.

source/blender/python/gpu/gpu_py_offscreen.c

@@ -246,7 +246,7 @@ static PyObject *bpygpu_offscreen_draw_view3d(BPyGPUOffScreen *self,
 
   BLI_assert(BKE_id_is_in_global_main(&scene->id));
 
-  depsgraph = BKE_scene_get_depsgraph(G_MAIN, scene, view_layer, true);
+  depsgraph = BKE_scene_ensure_depsgraph(G_MAIN, scene, view_layer);
 
   rv3d_mats = ED_view3d_mats_rv3d_backup(region->regiondata);