drm/nouveau: handle pci/tegra drm_dev_{alloc, register} from common code

drm/nouveau: handle pci/tegra drm_dev_{alloc, register} from common code

Unify some more of the PCI/Tegra DRM driver init, both as a general
cleanup, and because a subsequent commit changes the pointer stored
via dev_set_drvdata(), and this allows the change to be made in one
place.

Signed-off-by: Ben Skeggs <bskeggs@nvidia.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240726043828.58966-3-bskeggs@nvidia.com