drm/i915/gt: Push the GPU cancellation to the backend

Upon unregistering the user interface, we mark the GPU as wedged to
ensure we push no new work to the GPU, and to flush all current work
from the GPU. Move this call to the GT backend.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andi Shyti <andi.shyti@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200221235135.2883006-1-chris@chris-wilson.co.uk