x86/amd_nb: Clean up early_is_amd_nb()

The check for early_is_amd_nb() is only useful for systems with GART or
the NB_CFG register.

Zen-based systems (both AMD and Hygon) have neither, so return early for
them.

Signed-off-by: Yazen Ghannam <yazen.ghannam@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20241206161210.163701-4-yazen.ghannam@amd.com