This allows us to use fewer bits in the mode structure, leaving room for
future work while allowing more stereo layouts types than we could have
ever dreamt of.

I also exposed the previously private DRM_MODE_FLAG_3D_MASK to set in
stone that we are using 5 bits for the stereo layout enum, reserving 32
values.

Even with that reservation, we gain 3 bits from the previous encoding.

The code adding the mandatory stereo modes needeed to be adapted as it was
relying or being able to or stereo layouts together.
Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>