drm/i915: Fix debugfs PDP register dump

Iterate over all the PDP registers instead of just printing PDP0 four
times in gen8 PPGTT debugfs info.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>