drm/bridge: analogix-anx78xx: Remove duplicate NULL check

drm/bridge: analogix-anx78xx: Remove duplicate NULL check

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

Cc: Archit Taneja <architt@codeaurora.org>
Cc: Andrzej Hajda <a.hajda@samsung.com>
Cc: Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
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/20171031142149.32512-6-andriy.shevchenko@linux.intel.com
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>