drm: remove bus->get_irq implementations

Now that they're all unused we can get rid of them, including the
dummy version in drm_usb.c.

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