KVM: x86/mmu: replace direct_map with root_role.direct

direct_map is always equal to the direct field of the root page's role:

- for shadow paging, direct_map is true if CR0.PG=0 and root_role.direct is
copied from cpu_role.base.direct

- for TDP, it is always true and root_role.direct is also always true

- for shadow TDP, it is always false and root_role.direct is also always
false

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>