x86-64: move unlazy_fpu() into lazy cpu state part of context switch

Make sure that unlazy_fpu()'s stts gets batched along with the other
cpu state changes during context switch.  (32-bit already does this.)

This makes sure it gets batched when running paravirtualized.

[ Impact: optimise paravirtual FPU context switch ]

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Alok Kataria <akataria@vmware.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>