drm/i915: Fix fbdev unload sequence

First thing we need to do is unregister the fbdev instance, but we
can't just go ahead and kfree it. That must wait until the hotplug and
polling work are stopped, since they can race with the with the
teardown. That means we need to split up the fbdev teardown into the
unregister part and the cleanup part.

I originally suspected that this was broken in one of the unload
shuffles, but on closer inspection the oldest sequence I've dug out
also gets this wrong. Just not quite so badly.

I've run drv_module_reload a few hundred times and it's rock solid
compared to insta-death beforehand. This bug seems to have been
uncovered by

commit 88be58be
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Jul 6 15:00:19 2017 +0200

    drm/i915/fbdev: Always forward hotplug events

But the effect of that seems to only be to increase the race window
enough to make it blow up easier. I'm not exactly clear on what's
going on there ...

v2: Fix whitespace and use fetch_and_zero (Chris).

Testcase: igt/drv_module_reload
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101791


Cc: martin.peres@free.fr
Cc: chris@chris-wilson.co.uk
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170714224656.6431-2-daniel.vetter@ffwll.ch