x86/apic: KVM: Move lapic get/set helpers to common code

x86/apic: KVM: Move lapic get/set helpers to common code

Move the apic_get_reg(), apic_set_reg(), apic_get_reg64() and
apic_set_reg64() helper functions to apic.h in order to reuse them in the
Secure AVIC guest APIC driver in later patches to read/write registers
from/to the APIC backing page.

No functional change intended.

Signed-off-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Acked-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20250709033242.267892-12-Neeraj.Upadhyay@amd.com
Signed-off-by: Sean Christopherson <seanjc@google.com>