drm/i915: Move sseu debugfs under gt/

In line with what happened for other gt-related features, move the sseu
debugfs files under gt/.

The sseu_status debugfs has also been kept at the top level as we do
have tests that use it; it will be removed once we teach the tests to
look into the new path.

Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Andi Shyti <andi.shyti@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/20200708003952.21831-10-daniele.ceraolospurio@intel.com