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