drm/nouveau: store nvkm_device pointer in nouveau_drm

drm/nouveau: store nvkm_device pointer in nouveau_drm

There's various different places in the drm code that get at the
nvkm_device via various creative (and not very type-safe) means.

One of those being via nvif_device.object.priv.

Another patch series is going to entirely remove the ioctl-like
interfaces beween the drm code and nvkm, and that field will no
longer exist.

This provides a safer replacement for accessing the nvkm_device,
and will used more in upcoming patches to cleanup other cases.

v2:
- fixup printk macros to not oops if used before client ctor

Signed-off-by: Ben Skeggs <bskeggs@nvidia.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240726043828.58966-6-bskeggs@nvidia.com