ALSA: hda - Add keep_eapd_on flag to generic parser

VT1802 codec seems to reset EAPD of other pins in the hardware level,
and this was another reason of the silent headphone output on some
machines.  As a workaround, introduce a new flag indicating to keep
the EPAD on to the generic parser, and set it in patch_via.c.

Reported-by: Alex Riesen <raa.lkml@gmail.com>
Cc: <stable@vger.kernel.org> [v3.9]
Signed-off-by: Takashi Iwai <tiwai@suse.de>