drm/i915/gt: Avoid using the GPU before initialisation

Mark the GT as wedged so that we are not tempted to use it prior to
initialisation.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andi Shyti <andi.shyti@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191229183153.3719869-3-chris@chris-wilson.co.uk