drm: Remove drm_driver->set_busid hook

The only special-case is pci devices, and we can easily handle this in
the core. Do so and drop a pile of boilerplate from drivers.

Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170524145212.27837-5-daniel.vetter@ffwll.ch