drm/vmwgfx: Implement basic pm operations.

Currently we really only support S3, since the device doesn't support
saving of the 3D state.

On S3/S4, move all buffer objects to swappable memory and take down
GMR bindings. We need to do that from a PM notifier since we can't
do persistant memory allocations from the standard PM callbacks.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>