drm/i2c/sil164: Remove duplicate NULL check

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

Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171031142149.32512-7-andriy.shevchenko@linux.intel.comSigned-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

drivers/gpu/drm/i2c/sil164_drv.c

@@ -326,8 +326,7 @@ sil164_encoder_destroy(struct drm_encoder *encoder)
 {
 	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);