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>