drm/i915: Don't display nonsensical values in i915_ddb_info on gen < 9

When playing around with debugfs and a HSW machine I noticed that we
were displaying some garbled value in i915_ddb_info. This debugfs file
is only meaningful for gen9+, so don't display anything on earlier
platforms.

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