drm/amdgpu: use drm_gem_private_object_init

We use our own backing store and don't need the shmem file.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>