Get LA57 from the role_regs, which are initialized from the vCPU even
though TDP is enabled, instead of pulling the value directly from the
vCPU when computing the guest's root_level for TDP MMUs.  Note, the check
is inside an is_long_mode() statement, so that requirement is not lost.

Use role_regs even though the MMU's role is available and arguably
"better".  A future commit will consolidate the guest root level logic,
and it needs access to EFER.LMA, which is not tracked in the role (it
can't be toggled on VM-Exit, unlike LA57).

Drop is_la57_mode() as there are no remaining users, and to discourage
pulling MMU state from the vCPU (in the future).

No functional change intended.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622175739.3610207-41-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>