drm/vgem: Add a dummy platform device

The vgem driver is currently registered independent of any actual
device. Some usage of the dmabuf APIs require an actual device structure
to do anything. Register a dummy platform device for use with dmabuf.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1493923548-20878-2-git-send-email-labbott@redhat.com