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>

diff --cc drivers/gpu/drm/drm_pci.c
Simple merge

diff --cc drivers/gpu/drm/drm_platform.c
Simple merge

diff --cc drivers/gpu/drm/drm_usb.c
Simple merge

diff --cc drivers/gpu/drm/tegra/bus.c
Simple merge

diff --cc include/drm/drmP.h
Simple merge