KVM: x86: Move get_cs_db_l_bits() helper to SVM

Move kvm_get_cs_db_l_bits() to SVM and rename it appropriately so that
its svm_x86_ops entry can be filled via kvm-x86-ops, and to eliminate a
superfluous export from KVM x86.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220128005208.4008533-16-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>