bochs: use drm_fb_helper_set_suspend_unlocked in suspend/resume

The "initialized" member is going away. suspend/resume still works (even
if bochsfb_create is forced to fail).

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Link: http://patchwork.freedesktop.org/patch/msgid/20180906221810.20170-2-peter@lekensteyn.nl
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>