x86/power: Optimize C3 entry on Centaur CPUs

For new Centaur CPUs the ucode will take care of the preservation of cache coherence
between CPU cores in C-states regardless of how deep the C-states are. So, it is not
necessary to flush the caches in software befor entering C3. This useless operation
will cause performance drop for the cores which share some caches with the idling core.

Signed-off-by: David Wang <davidwang@zhaoxin.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Pavel Machek <pavel@ucw.cz>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: brucechang@via-alliance.com
Cc: cooperyan@zhaoxin.com
Cc: len.brown@intel.com
Cc: linux-pm@kernel.org
Cc: qiyuanwang@zhaoxin.com
Cc: rjw@rjwysocki.net
Cc: timguo@zhaoxin.com
Link: http://lkml.kernel.org/r/1545900110-2757-1-git-send-email-davidwang@zhaoxin.com
[ Tidy up the comment. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>