GPU-DRM-OMAP: Delete unnecessary checks before two function calls

The following functions test whether their argument is NULL and then
return immediately.
* backlight_device_unregister
* drm_gem_object_unreference_unlocked

Thus the test around the calls is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/72ff4670-4f3d-c7ed-4f37-e49f2bbc7aba@users.sourceforge.net