drm/i915/skl: Augment the latency debugfs files for SKL

v2: Use the gen >= 9 in the debugfs file condition (Ville)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>