drm/vgem: Convert to a struct drm_device subclass

With Laura's introduction of the fake platform device for importing
dmabuf, we add a second static that is logically tied to the vgem_device.
Convert vgem over to using the struct drm_device subclassing, so that
the platform device is stored inside its owner.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Laura Abbott <labbott@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170508132228.9509-1-chris@chris-wilson.co.uk