drm: Code stereo layouts as an enum rather than a bit field

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>