drm/bridge: ps8622: Delete an unnecessary check before backlight_device_unregister()

The backlight_device_unregister() function tests whether its argument
is NULL and then returns immediately.
Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/791089e4-201c-ad01-9c3b-f49835765177@users.sourceforge.net