drm: kill drm_bus->bus_type

Completely unused. Hooray, midlayer mistakes that didn't cause work to
undo!

v2: Rebase on top of the recent tegra changes which added a host1x drm
bus.

Reviewed-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>