drm: fix the addition of the side-by-side (half) flag for extra 3D modes

Ensure the side-by-side (half) flag is added to any existing flags when
adding modes from 3D_Structure_ALL.

Signed-off-by: Thomas Wood <thomas.wood@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>