x86/CPU/AMD: Rename init_amd_zn() to init_amd_zen_common()

Call it from all Zen init functions.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Link: http://lore.kernel.org/r/20231120104152.13740-7-bp@alien8.de