KVM: SVM: Drop redundant svm_sev_enabled() helper

Replace calls to svm_sev_enabled() with direct checks on sev_enabled, or
in the case of svm_mem_enc_op, simply drop the call to svm_sev_enabled().
This effectively replaces checks against a valid max_sev_asid with checks
against sev_enabled.  sev_enabled is forced off by sev_hardware_setup()
if max_sev_asid is invalid, all call sites are guaranteed to run after
sev_hardware_setup(), and all of the checks care about SEV being fully
enabled (as opposed to intentionally handling the scenario where
max_sev_asid is valid but SEV enabling fails due to OOM).

Reviewed by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210422021125.3417167-14-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>