KVM: x86: Make kvm_mtrr_valid() static now that there are no external users

Make kvm_mtrr_valid() local to mtrr.c now that it's not used to check the
validity of a PAT MSR value.

Reviewed-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20230511233351.635053-8-seanjc@google.com


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