drm: Revert "drm: Reject modes with more than 1 stereo flags set"

Now that the coding of stereo layout has changed from a bit field to an
enum, we need remove that check.

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>