KVM: x86: Drop superfluous kvm_set_pic_irq() => kvm_pic_set_irq() wrapper

KVM: x86: Drop superfluous kvm_set_pic_irq() => kvm_pic_set_irq() wrapper

Drop the superfluous and confusing kvm_set_pic_irq() => kvm_pic_set_irq()
wrapper, and instead wire up ->set() directly to its final destination.

Opportunistically move the declaration kvm_pic_set_irq() to irq.h to
start gathering more of the in-kernel APIC/IO-APIC logic in irq.{c,h}.

No functional change intended.

Acked-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20250611213557.294358-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>