x86/mce/amd: Init thresholding machinery only on relevant vendors

... and not unconditionally.

 [ bp: Add a new vendor_flags bit for that. ]

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200403161943.1458-3-bp@alien8.de