drm/i915: Use complete address space in true PPGTT

True PPGTT is capable of having a full address space, even if the system
has less allocated memory.

Note that aliasing PPGTT always aliases the GGTT and thus should remain
of the same size.

Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>