drm/nouveau: use GEM references instead of TTMs

TTM wants to get rid of the duplicate refcounting of the embedded GEM
object and its own reference count.

Hence, use of GEM object references where possible.

Also get rid of nouveau_bo_ref() and replace it with nouveau_bo_fini(),
which drops the initial reference we get from initializing a ttm_bo.

Reviewed-by: Ben Skeggs <bskeggs@nvidia.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240718165959.3983-4-dakr@kernel.org