x86/cpu: KVM: Move macro to encode PAT value to common header

Move pat/memtype.c's PAT() macro to msr-index.h as PAT_VALUE(), and use it
in KVM to define the default (Power-On / RESET) PAT value instead of open
coding an inscrutable magic number.

No functional change intended.

Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240605231918.2915961-3-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>