drm/nouveau: Fill out gem_object->resv

That way we can ditch our gem_prime_res_obj implementation. Since ttm
absolutely needs the right reservation object all the boilerplate is
already there and we just have to wire it up correctly.

Note that gem/prime doesn't care when we do this, as long as we do it
before the bo is registered and someone can call the handle2fd ioctl
on it.

Aside: ttm_buffer_object.ttm_resv could probably be ditched in favour
of always passing a non-NULL resv to ttm_bo_init(). At least for gem
drivers that would avoid having two of these, on in ttm_buffer_object
and the other in drm_gem_object, one just there for confusion.

Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: nouveau@lists.freedesktop.org
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190725132655.11951-3-daniel.vetter@ffwll.ch