drm/i915: Mark GEM wedged right after marking device unplugged

As soon as a device is considered unplugged, not only prevent pending
users from accessing the device structures but also cancel all their
pending requests so all consumed resources can be cleaned up as soon
as possible.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190406104034.31380-1-chris@chris-wilson.co.uk