x86/tlb: Move PCID helpers where they are used

Aside of the fact that they are used only in the TLB code, especially
having the comment close to the actual implementation makes a lot of
sense.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200421092600.145772183@linutronix.de