drm/i915/gen8: Flip the 48b switch

Use 48b addresses if hw supports it (i915.enable_ppgtt=3).
Update the sanitize_enable_ppgtt for 48 bit PPGTT mode.

Note, aliasing PPGTT remains 32b only.

v2: s/full_64b/full_48b/. (Akash)
v3: Add sanitize_enable_ppgtt changes until here. (Akash)
v4: Update param description (Chris)

Cc: Akash Goel <akash.goel@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>