drm/i915: Avoid circular locking from intel_fbdev_fini()

lockdep spots that the fb_info->lock takes the dev->struct_mutex during
init (due to the device probing) and so we can not hold
dev->struct_mutex when unregistering the framebuffer. Simply reverse the
order of initialisation during cleanup and so do the intel_fbdev_fini()
before the intel_modeset_cleanup.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>