drm/i2c/sil164: Remove duplicate NULL check

Since i2c_unregister_device() became NULL-aware we may remove duplicate
NULL check.

Cc: David Airlie <[e-mail]>
Cc: [e-mail]
Signed-off-by: Andy Shevchenko <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
Signed-off-by: Ville Syrjälä <[e-mail]>

drivers/gpu/drm/i2c/sil164_drv.c

···
 {
 	struct sil164_priv *priv = to_sil164_priv(encoder);
 
-	if (priv->duallink_slave)
-		i2c_unregister_device(priv->duallink_slave);
+	i2c_unregister_device(priv->duallink_slave);
 
 	kfree(priv);
 	drm_i2c_encoder_destroy(encoder);