platform: x86: Deletion of checks 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>
For msi-wmi.c:
Acked-by: Anisse Astier <anisse@astier.eu>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>