HDMI 1.4a defines a few layouts that we'd like to expose. This commits
add new modeinfo flags that can be used to list the supported stereo
layouts (when querying the list of modes) and to set a given stereo 3D
mode (when setting a mode).

v2: Add a drm_mode_is_stereo() helper
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>