KVM: arm64: Don't hide ID registers from userspace

ID registers are RAZ until they've been allocated a purpose, but
that doesn't mean they should be removed from the KVM_GET_REG_LIST
list. So far we only have one register, SYS_ID_AA64ZFR0_EL1, that
is hidden from userspace when its function, SVE, is not present.

Expose SYS_ID_AA64ZFR0_EL1 to userspace as RAZ when SVE is not
implemented. Removing the userspace visibility checks is enough
to reexpose it, as it will already return zero to userspace when
SVE is not present. The register already behaves as RAZ for the
guest when SVE is not present.

Fixes: 73433762 ("KVM: arm64/sve: System register context switch and access support")
Reported-by: 张东旭 <xu910121@sina.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org#v5.2+
Link: https://lore.kernel.org/r/20201105091022.15373-2-drjones@redhat.com