KVM: arm/arm64: Avoid attempting to load timer vgic state without a vgic

The timer optimization patches inadvertendly changed the logic to always
load the timer state as if we have a vgic, even if we don't have a vgic.

Fix this by doing the usual irqchip_in_kernel() check and call the
appropriate load function.

Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>