drm/i2c: tda998x: Remove duplicate NULL check

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

Cc: Russell King <linux@armlinux.org.uk>
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/20171031142149.32512-8-andriy.shevchenko@linux.intel.com


Acked-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>