drm/i915: set AUD_CONFIG N_value_index for DisplayPort
It should be programmed to "0" for HDMI or "1" for DisplayPort.
This enables DisplayPort audio for
- HP EliteBook 8460p
(whose BIOS does not set the N_value_index bit for us)
- DisplayPort monitor hot plugged after boot
(otherwise most BIOS will fill the N_value_index bit for us)
Tested-by: Robert Lemaire <rlemaire@suse.com>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>