[...]
[   38s]   LD [M] 
/home/abuild/rpmbuild/BUILD/nvidia-gfxG04-390.138/obj/default/390.138/nvidia-drm.o
[   38s]   MODPOST
/home/abuild/rpmbuild/BUILD/nvidia-gfxG04-390.138/obj/default/390.138/Module.symvers
[   38s] FATAL: modpost: GPL-incompatible module nvidia-uvm.ko uses GPL-only
symbol 'radix_tree_preloads'

Seems this came with git commit id cfa6705d89b65 between 5.7 and 5.8-rc1

commit cfa6705d89b6562f79c40c249f8d94073c4276e4
Author: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date:   Wed May 27 22:11:14 2020 +0200

    radix-tree: Use local_lock for protection

[...]
@@ -58,12 +58,10 @@ struct kmem_cache *radix_tree_node_cachep;
 /*
  * Per-cpu pool of preloaded nodes
  */
-struct radix_tree_preload {
-       unsigned nr;
-       /* nodes->parent points to next preallocated node */
-       struct radix_tree_node *nodes;
+DEFINE_PER_CPU(struct radix_tree_preload, radix_tree_preloads) = {
+       .lock = INIT_LOCAL_LOCK(lock),
 };
-static DEFINE_PER_CPU(struct radix_tree_preload, radix_tree_preloads) = { 0,
};
+EXPORT_PER_CPU_SYMBOL_GPL(radix_tree_preloads);

 static inline struct radix_tree_node *entry_to_node(void *ptr)
 {
[...]