drm: Make exposing stereo modes a per-connector opt-in

Just like with interlaced or double scan modes, make stereo modes a
per-connector opt-in to give a chance to driver authors to make it work
before enabling it.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
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>