x86/oprofile: Rework and simplify nmi_cpu_setup()

This patch removes the function nmi_save_registers(). Per-cpu code is
now executed only in the function nmi_cpu_setup().  Also, it renames
the per-cpu function nmi_restore_registers() to
nmi_cpu_restore_registers().

Signed-off-by: Robert Richter <robert.richter@amd.com>