ALSA: hda/realtek - Add path active flag

... and rewrite the initialization of output paths as a generic
function that is applicable for both i/o directions.

The new flag, active, is introduced to each nid_path entry.  This
indicates whether the given path is active, and it's used for checking
whether a certain widget can be turned off or changed when a path is
no longer used or newly enabled.

It's still used only in the output paths.  More wider adaption for
input and loopback paths will be achieved in the later patch.

Signed-off-by: Takashi Iwai <tiwai@suse.de>