KVM: x86/mmu: constify uses of struct kvm_mmu_role_regs

struct kvm_mmu_role_regs is computed just once and then accessed.  Use
const to make this clearer, even though the const fields of struct
kvm_mmu_role_regs already prevent (or make it harder...) to modify
the contents of the struct.

Reviewed-by: David Matlack <dmatlack@google.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>