drm/i915/sdvo: Grab mode_config.mutex during LVDS init to avoid WARNs

drm_mode_probed_add() is unhappy about being called w/o
mode_config.mutex. Grab it during LVDS fixed mode setup
to silence the WARNs.

Cc: stable@vger.kernel.org
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/7301
Fixes: aa2b8807 ("drm/i915/sdvo: Fix multi function encoder stuff")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221026101134.20865-4-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>

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

@@ -2908,8 +2908,12 @@ intel_sdvo_lvds_init(struct intel_sdvo *intel_sdvo, int device)
 	intel_panel_add_vbt_sdvo_fixed_mode(intel_connector);
 
 	if (!intel_panel_preferred_fixed_mode(intel_connector)) {
+		mutex_lock(&i915->drm.mode_config.mutex);
+
 		intel_ddc_get_modes(connector, &intel_sdvo->ddc);
 		intel_panel_add_edid_fixed_modes(intel_connector, false);
+
+		mutex_unlock(&i915->drm.mode_config.mutex);
 	}
 
 	intel_panel_init(intel_connector);