drm/i915/cnl: Add slice and subslice information to debugfs.

A missing part to EU slice power gating is the
debugfs interface. This patch actually should have been
squashed to the initial EU slice power gating one.

v2: Initial patch was merged without this part.

Fixes: c7ae7e9a ("drm/i915/cnl: Configure EU slice power gating.")
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170809200702.11236-1-rodrigo.vivi@intel.com