arm64/kvm: Make use of the system wide safe values

Use the system wide safe value from the new API for safer
decisions

Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Christoffer Dall <christoffer.dall@linaro.org>
Cc: kvmarm@lists.cs.columbia.edu
Signed-off-by: Suzuki K. Poulose <suzuki.poulose@arm.com>
Acked-by: Christoffer Dall <christoffer.dall@linaro.org>
Tested-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>