drm/i915: Use drm_dev_unplug()

The driver does not currently support unbinding from a device which is
in use.  Since open file descriptors may still be pointing into kernel
memory where the device structures used to be, entirely correct kernel
panics protect the driver from being unbound as we should not be
unbinding it before those dangling pointers have been made safe.

According to the documentation found inside drivers/gpu/drm/drm_drv.c,
drm_dev_unplug() should be used instead of drm_dev_unregister() in
order to make a device inaccessible to users as soon as it is unpluged.
Follow that advice to make those possibly dangling pointers safe,
protected by DRM layer from a user who is otherwise left pointing into
possibly reused kernel memory after the driver has been unbound from
the device.  Once done, also cancel inflight operations immediately by
calling i915_gem_set_wedged().

Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190405130235.7707-2-janusz.krzysztofik@linux.intel.com

drivers/gpu/drm/i915/i915_drv.c

@@ -1760,7 +1760,7 @@ static void i915_driver_unregister(struct drm_i915_private *dev_priv)
 	i915_pmu_unregister(dev_priv);
 
 	i915_teardown_sysfs(dev_priv);
-	drm_dev_unregister(&dev_priv->drm);
+	drm_dev_unplug(&dev_priv->drm);
 
 	i915_gem_shrinker_unregister(dev_priv);
 }