KVM: arm64: Calculate hyp VA size only once

KVM: arm64: Calculate hyp VA size only once

Calculate the hypervisor's VA size only once to maintain consistency
between the memory layout and MMU initialization logic. Previously the
two would be inconsistent when the kernel is configured for less than
IDMAP_VA_BITS of VA space.

Signed-off-by: Petteri Kangaslampi <pekangas@google.com>
Tested-by: Vincent Donnefort <vdonnefort@google.com>
Link: https://patch.msgid.link/20260113194409.2970324-2-pekangas@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>