drm/i915/debugfs: reuse max slice/subslices already stored in sseu

Now that we have that information in topology fields, let's just reuse it.

v2: Style tweaks (Tvrtko)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180306122857.27317-3-lionel.g.landwerlin@intel.com