drm/virtio: Drop deprecated load/unload initialization

Move the code around so the driver is probed the bus
.probe and removed from the bus .remove callbacks.
This commit is just a cleanup and shouldn't affect
functionality.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190108145930.15080-1-ezequiel@collabora.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>