drm: Reject modes with more than 1 stereo flags set

When setting a stereo 3D mode, there can be only one bit set describing
the layout of the frambuffer(s). So reject invalid modes early.

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>