drm/i915: Actually capture PP_DIR_BASE on error

I have been seeing this for a long time, but ignored it because it's
typically not terribly important. Recently, I really needed this info,
and it was garbage. Proof that I should have fixed it sooner. Originally
wrong from:

commit 6c7a01ec
Author: Ben Widawsky <benjamin.widawsky@intel.com>
Date:   Thu Jan 30 00:19:40 2014 -0800

    drm/i915: Capture PPGTT info on error capture

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>