drm/nouveau: init the base GEM fields for internal BOs

TTMs buffer objects are based on GEM objects for quite a while
and rely on initializing those fields before initializing the TTM BO.

Nouveau now doesn't init the GEM object for internally allocated BOs,
so make sure that we at least initialize some necessary fields.

Signed-off-by: Christian König <christian.koenig@amd.com>
Tested-by: Mikko Perttunen <mperttunen@nvidia.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210609172902.1937-1-christian.koenig@amd.com