drm: Clean up drm_dev_unplug

Use drm_dev_unregister to unregister the interfaces, which also allows
us to simplify the open_count == 0 case.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170802115604.12734-5-daniel.vetter@ffwll.ch