powertop: fix segfault in dump mode

powertop: fix segfault in dump mode

The cause is: in dump mode, setup_windows() is not invoked so the
*_window variables, like cstate_window, are still NULL; later in
main() -> show_cstates(), the functions, like wrefresh(), will trigger
segfault.

After discussing in PowerTop mailling list, I think the right solution is
we should stub out the ncurses calls in dump mode.

(The patch was also sent to upstream PowerTop mailling list.)

This fixes [BUGID #195]:
Bug 195 - [sdk] powertop segfault when running with dump option

Signed-off-by: Dexuan Cui <dexuan.cui@intel.com>