drm/nouveau/kms/nv50-: Initialize core channel in nouveau_display_create()

We'll need the core channel initialized and ready by the time that we
start creating modesetting objects, so that we can call the
NV507D_GET_CAPABILITIES method to make the hardware expose it's
modesetting capabilities for later probing.

So, when loading the driver prepare the core channel from within
nouveau_display_create(). Everywhere else, we initialize the core
channel during resume.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>