We can have exactly 4GB sized ppgtt with 32bit system.
size_t is inadequate for this.

v2: Convert a lot more places (Daniel)
Signed-off-by: NMika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: NMichel Thierry <michel.thierry@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>