KVM: arm64: Check the pgt instead of the pgd when modifying page-table

In preparation for removing the 'pgd' field of 'struct kvm_s2_mmu',
update the few remaining users to check the 'pgt' field instead.

Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Quentin Perret <qperret@google.com>
Link: https://lore.kernel.org/r/20200911132529.19844-19-will@kernel.org