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: NMichel Thierry <michel.thierry@intel.com>
Reviewed-by: NMika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>