x86: fix: do not run code in amd_bus.c on non-AMD CPUs

Jan Beulich wrote:

> Even worse - this would even try to access the MSR on non-AMD CPUs
> (currently probably prevented just by the fact that only AMD ones use
> family values of 0x10 or higher).

This patch adds cpu vendor check to the postcore_initcalls.

Reported-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Robert Richter <robert.richter@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>