KVM: arm64: Keep mdcr_el2's value as set by __init_el2_debug

__init_el2_debug configures mdcr_el2 at initialization based on,
among other things, available hardware support. Trap deactivation
doesn't check that, so keep the initial value.

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/20210817081134.2918285-8-tabba@google.com