[POWERPC] Clean up pmac_backlight_unblank in oops path

Move pmac_backlight_unblank into its own function and only take the
pmac_backlight_mutex when we are on a pmac for that added bit of
paranoia.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>