drm/edid: request HDMI underscan by default

Working with HDMI TVs is a real pain as they tend to overscan by
default, meaning that the pixels around the edge of the framebuffer
are not displayed. This is well explained here:
http://mjg59.dreamwidth.org/8705.html

There is a bit in the HDMI info frame that can request that the
remote display shows the full pixel data ("underscan"). For the
remote display, the HDMI spec states that this is optional - it
doesn't have to listen. That means that most TVs will probably ignore
this.

But, maybe there are a handful of TVs for which this would help
the situation. As we live in a digital world, ask the remote
display not to overscan by default.

Signed-off-by: Daniel Drake <drake@endlessm.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_edid.c

@@ -3599,6 +3599,7 @@ drm_hdmi_avi_infoframe_from_display_mode(struct hdmi_avi_infoframe *frame,
 
 	frame->picture_aspect = HDMI_PICTURE_ASPECT_NONE;
 	frame->active_aspect = HDMI_ACTIVE_ASPECT_PICTURE;
+	frame->scan_mode = HDMI_SCAN_MODE_UNDERSCAN;
 
 	return 0;
 }