x86/mce/AMD: Carve out threshold block preparation

mce_amd_feature_init() was getting pretty fat, carve out the
threshold_block setup into a separate function in order to
simplify flow and make it more understandable.

No functionality change.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Aravind Gopalakrishnan <Aravind.Gopalakrishnan@amd.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Link: http://lkml.kernel.org/r/1453750913-4781-8-git-send-email-bp@alien8.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>