arm64: KVM: Reorder system register restoration and stage-2 activation

In order to prepare for handling erratum 1319367, we need to make
sure that all system registers (and most importantly the registers
configuring the virtual memory) are set before we enable stage-2
translation.

This results in a minor reorganisation of the load sequence, without
any functional change.

Reviewed-by: James Morse <james.morse@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>