drm/i915/dp_mst: Reset intel_dp->link_trained during disabling

Reset the flag indicating an active link after disabling an MST link,
similarly to how this is done for SST outputs. This avoids trying to
retrain an MST link while its disabled.
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240610164933.2947366-16-imre.deak@intel.com

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

@@ -983,6 +983,9 @@ static void intel_mst_disable_dp(struct intel_atomic_state *state,
 	drm_dbg_kms(&i915->drm, "active links %d\n",
 		    intel_dp->active_mst_links);
 
+	if (intel_dp->active_mst_links == 1)
+		intel_dp->link_trained = false;
+
 	intel_hdcp_disable(intel_mst->connector);
 
 	intel_dp_sink_disable_decompression(state, connector, old_crtc_state);