KVM: arm64: Restore mdcr_el2 from vcpu

On deactivating traps, restore the value of mdcr_el2 from the
newly created and preserved host value vcpu context, rather than
directly reading the hardware register.

Up until and including this patch the two values are the same,
i.e., the hardware register and the vcpu one. A future patch will
be changing the value of mdcr_el2 on activating traps, and this
ensures that its value will be restored.

No functional change intended.

Signed-off-by: Fuad Tabba <tabba@google.com>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210817081134.2918285-7-tabba@google.com