drm/udl: Remove dummy busid callback

Since

commit ca8e2ad7
Author: Thierry Reding <treding@nvidia.com>
Date:   Fri Apr 11 15:23:00 2014 +0200

    drm: Introduce drm_dev_set_unique()

the ->set_busid callback is optional. On top of that the udl one isn't
really fully compliant with the drm uabi, but since only modesetting
ever binds to it (there's no 3d accel on udl) it doesn't matter.

Still, can't harm to aling and use the default used by everyone else.

Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170524145212.27837-4-daniel.vetter@ffwll.ch