drm/xe: Fix build with CONFIG_MODULES=n

When building with CONFIG_MODULES=n, the __exit functions are dropped.
However our init functions may call them for error handling, so they are
not good candidates for the exit sections.

Fix this error reported by 0day:

	ld.lld: error: relocation refers to a symbol in a discarded section: xe_configfs_exit
	>>> defined in vmlinux.a(drivers/gpu/drm/xe/xe_configfs.o)
	>>> referenced by xe_module.c
	>>>               drivers/gpu/drm/xe/xe_module.o:(init_funcs) in archive vmlinux.a

This is the only exit function using __exit. Drop it to fix the build.

Cc: Riana Tauro <riana.tauro@intel.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202506092221.1FmUQmI8-lkp@intel.com/
Fixes: 16280de ("drm/xe: Add configfs to enable survivability mode")
Reviewed-by: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Link: https://lore.kernel.org/r/20250912-fix-nomodule-build-v1-1-d11b70a92516@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>

Author: Lucas De Marchi

drivers/gpu/drm/xe/xe_configfs.c

@@ -990,7 +990,7 @@ int __init xe_configfs_init(void)
 	return 0;
 }
 
-void __exit xe_configfs_exit(void)
+void xe_configfs_exit(void)
 {
 	configfs_unregister_subsystem(&xe_configfs);
 	mutex_destroy(&xe_configfs.su_mutex);