drm/i915/gt: Take responsibility for engine->release as the last step

In order to avoid a double cleanup on error, take ownership of
engine->release past the point of no [error] return.

Reported-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Fixes: e26b6d43

 ("drm/i915/gt: Pull GT initialisation under intel_gt_init()")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Tested-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200107143118.3288995-1-chris@chris-wilson.co.uk