drm/nouveau: Initialize GEM object before TTM object

TTM assumes that drivers initialize the embedded GEM object before
calling the ttm_bo_init() function. This is not currently the case
in the Nouveau driver. Fix this by splitting up nouveau_bo_new()
into nouveau_bo_alloc() and nouveau_bo_init() so that the GEM can
be initialized before TTM BO initialization when necessary.

Fixes: b96f3e7c

 ("drm/ttm: use gem vma_node")
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190814093524.GA31345@ulmo