KVM: arm64: Fix host's ZCR_EL2 restore on nVHE

We re-enter the EL1 host with CPTR_EL2.TZ set in order to
be able to lazily restore ZCR_EL2 when required.

However, the same CPTR_EL2 configuration also leads to trapping
when ZCR_EL2 is accessed from EL2. Duh!

Clear CPTR_EL2.TZ *before* writing to ZCR_EL2.

Fixes: beed0906 ("KVM: arm64: Trap host SVE accesses when the FPSIMD state is dirty")
Reported-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>