drm/edid: Expose mandatory stereo modes for HDMI sinks

drm/edid: Expose mandatory stereo modes for HDMI sinks

For now, let's just look at the 3D_present flag of the CEA HDMI vendor
block to detect if the sink supports a small list of then mandatory 3D
formats.

See the HDMI 1.4a 3D extraction for detail:
  http://www.hdmi.org/manufacturer/specification.aspx

v2: Rename freq to vrefresh, make the mandatory structure a bit more
    compact, fix some white space issues and add a couple of const
    (Ville Syrjälä)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>