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.com


Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>