bochs: convert to drm_dev_register

The drm_get_pci_dev API is deprecated, replace it by drm_dev_register.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Link: http://patchwork.freedesktop.org/patch/msgid/20180906221810.20170-4-peter@lekensteyn.nl
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>