drm/nouveau/drm/nouveau: tegra: Call nouveau_drm_device_init()

As part of commit cfea88a4

 ("drm/nouveau: Start using new drm_dev
initialization helpers"), the initialization of the Nouveau DRM device
was reworked and along the way the platform driver initialization was
left incomplete. Add a call to nouveau_drm_device_init() to make sure
all of the structures are properly initialized.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>