support for platform devices

Upcoming mobile Kepler GPUs (such as GK20A) use the platform bus instead
of PCI to which Nouveau is tightly dependent. This patch allows Nouveau
to handle platform devices by:

- abstracting PCI-dependent functions that were typically used for
  resource querying and page mapping,
- introducing a nv_device_is_pci() function that allows to make
  PCI-dependent code conditional,
- providing a nouveau_drm_platform_probe() function that takes a GPU
  platform device to be probed.

Core code as well as engine/subdev drivers are updated wherever possible
to make use of these functions. Some older drivers are too dependent on
PCI to be properly updated, but all newer code on which future chips may
depend should at least be runnable with platform devices.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>