drm/virtgpu: Delete unnecessary checks before drm_gem_object_unreference_unlocked()

drm/virtgpu: Delete unnecessary checks before drm_gem_object_unreference_unlocked()

The drm_gem_object_unreference_unlocked() function tests whether
its argument is NULL and then returns immediately.
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/5e3dafff-72bb-e616-e84a-368f78fa66b1@users.sourceforge.net