drm/udl: move to embedding drm device inside udl device.

This should help with some of the lifetime issues, and move us away
from load/unload.

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190405031715.5959-4-airlied@gmail.com