drm/i915/mso: using joiner is not possible with eDP MSO

It's not possible to use the joiner at the same time with eDP MSO. When
a panel needs MSO, it's not optional, so MSO trumps joiner.

v3: Only change intel_dp_has_joiner(), leave debugfs alone (Ville)

Fixes: bc71194e ("drm/i915/edp: enable eDP MSO during link training")
Cc: <stable@vger.kernel.org> # v5.13+
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/1668Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240614142311.589089-1-jani.nikula@intel.comSigned-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/display/intel_dp.c

@@ -465,6 +465,10 @@ bool intel_dp_has_joiner(struct intel_dp *intel_dp)
 	struct intel_encoder *encoder = &intel_dig_port->base;
 	struct drm_i915_private *dev_priv = to_i915(encoder->base.dev);
 
+	/* eDP MSO is not compatible with joiner */
+	if (intel_dp->mso_link_count)
+		return false;
+
 	return DISPLAY_VER(dev_priv) >= 12 ||
 		(DISPLAY_VER(dev_priv) == 11 &&
 		 encoder->port != PORT_A);