drm: bridge: cdns-mhdp8546: Fix missing mutex unlock on error path

Add missing mutex unlock before returning from the error path in
cdns_mhdp_atomic_enable().

Fixes: 935a92a1c400 ("drm: bridge: cdns-mhdp8546: Fix possible null pointer dereference")
Reported-by: Hulk Robot <[e-mail]>
Signed-off-by: Qi Xi <[e-mail]>
Reviewed-by: Luca Ceresoli <[e-mail]>
Reviewed-by: Dmitry Baryshkov <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Luca Ceresoli <[e-mail]>

drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c

···
 	mhdp_state = to_cdns_mhdp_bridge_state(new_state);
 
 	mhdp_state->current_mode = drm_mode_duplicate(bridge->dev, mode);
-	if (!mhdp_state->current_mode)
-		return;
+	if (!mhdp_state->current_mode) {
+		ret = -EINVAL;
+		goto out;
+	}
 
 	drm_mode_set_name(mhdp_state->current_mode);