drm: Remove drm_device->virtdev

This is a leftover from the drm_bus days, where we've had a
bus-specific device type for every bus type in drm_device. Except for
pci (which we can't remove because dri1 drivers) this is all gone. And
the virt driver also doesn't really need it, dev_to_virtio works
perfectly fine.

Cc: David Airlie <airlied@linux.ie>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: virtualization@lists.linux-foundation.org
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170524145212.27837-3-daniel.vetter@ffwll.ch