We have no bigjoiner support in the MST code, so .mode_valid()
pretending otherwise is just going to result black screens for
users. Reject any mode that needs the joiner.

Cc: stable@vger.kernel.org
Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Fixes: d51f25eb ("drm/i915: Add DSC support to MST path")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231127145028.4899-3-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>