KVM: arm64: Reorder handling of invariant sysregs from userspace

In order to allow some further refactor of the sysreg helpers,
move the handling of invariant sysreg to occur before we handle
all the other ones.

Reviewed-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>