x86/amd_nb: Check vendor in AMD-only functions

Exit early in functions which are meant to run on AMD only but which get
run on different vendor (VMs, etc).

 [ bp: rewrite commit message. ]

Signed-off-by: Pu Wen <puwen@hygon.cn>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: bhelgaas@google.com
Cc: tglx@linutronix.de
Cc: mingo@redhat.com
Cc: hpa@zytor.com
Cc: x86@kernel.org
Cc: thomas.lendacky@amd.com
Cc: helgaas@kernel.org
Link: https://lkml.kernel.org/r/487d8078708baedaf63eb00a82251e228b58f1c2.1537885177.git.puwen@hygon.cn