drm/amdgpu: Delete an unnecessary check 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 call is not needed.
This issue was detected by using the Coccinelle software.
Reviewed-by: Christian König <christian.koenig@amd.com> Signed-off-by: Markus Elfring <elfring@users.sourceforge.net> Signed-off-by: Alex Deucher <alexander.deucher@amd.com>