x86: misc clean up after the percpu update

Do the following cleanups:

* kill x86_64_init_pda() which now is equivalent to pda_init()

* use per_cpu_offset() instead of cpu_pda() when initializing
  initial_gs

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/xen/enlighten.c

@@ -1645,7 +1645,7 @@ asmlinkage void __init xen_start_kernel(void)
 #ifdef CONFIG_X86_64
 	/* Disable until direct per-cpu data access. */
 	have_vcpu_info_placement = 0;
-	x86_64_init_pda();
+	pda_init(0);
 #endif
 
 	xen_smp_init();