drm/i915/gt: Ignore incomplete engines after init failure

Do not expose incomplete engines to the user after we fail to setup the
GT.

Fixes: e6ba7648 ("drm/i915: Remove i915->kernel_context")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andi Shyti <andi.shyti@intel.com>
Acked-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191226191237.2654510-1-chris@chris-wilson.co.uk