drm/i915: Sanitize the enable_ppgtt module option once

Otherwise we'll end up spamming dmesg on every context creation on snb
with vt-d enabled. This regression was introduced in

commit 246cbfb5
Author: Ben Widawsky <benjamin.widawsky@intel.com>
Date:   Fri Dec 6 14:11:14 2013 -0800

    drm/i915: Reorganize intel_enable_ppgtt

As the i915.enable_ppgtt is read-only it cannot be changed after the
module is loaded and so we can perform an early sanitization of the
values.

v2:
- Add comment and pimp commit message (Chris)
- Use the param consistently (Jani)

v3:
- Fix init sequence on pre-gen6 by moving the sanitize_ppgtt call to
  gtt_init. Fixes boot hangs on pre-gen6.
- Add a debug output for the sanitize ppgtt mode.

References: https://lkml.org/lkml/2014/4/17/599
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=77916


Cc: Alessandro Suardi <alessandro.suardi@gmail.com>
Cc: Ben Widawsky <ben@bwidawsk.net>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>