drm/i915: add a "force-dvi" HDMI audio mode

When HDMI-DVI converter is used, it's not only necessary to turn off
audio, but also to disable HDMI_MODE_SELECT and video infoframe. Since
the DVI mode is mainly tied to audio functionality from end user POV,
add a new "force-dvi" audio mode:

	xrandr --output HDMI1 --set audio force-dvi

Note that most users won't need to set this and happily rely on the EDID
based DVI auto detection.

Reported-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_modes.c

@@ -84,6 +84,7 @@ int intel_ddc_get_modes(struct drm_connector *connector,
 }
 
 static const char *force_audio_names[] = {
+	"force-dvi",
 	"off",
 	"auto",
 	"on",
@@ -106,7 +107,8 @@ intel_attach_force_audio_property(struct drm_connector *connector)
 			return;
 
 		for (i = 0; i < ARRAY_SIZE(force_audio_names); i++)
-			drm_property_add_enum(prop, i, i-1, force_audio_names[i]);
+			drm_property_add_enum(prop, i, i-2,
+					      force_audio_names[i]);
 
 		dev_priv->force_audio_property = prop;
 	}