drm/i915: Remove debugfs/i915_ppgtt_info

The information presented here is not relevant to current development.
We can either use the context information, but more often we want to
inspect the active gpu state.

The ulterior motive is to eradicate dev->filelist.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181227121549.29139-1-chris@chris-wilson.co.uk