drm/i915: limit PPGTT size to 2GB in 32-bit platforms

We already set this limit for the GGTT.

This is a temporary patch until a full replacement of size_t variables
(inadequate in 32-bit kernel) is in place.

Regression from:
	commit a4e0bedc
	Author: Michel Thierry <michel.thierry@intel.com>
	Date:   Wed Apr 8 12:13:35 2015 +0100

		drm/i915: Use complete address space in true PPGTT

v2: Prettify code and explain why this is needed. (Chris)
v3: Don't hide the compilation warning in 32-bit. (Chris)

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Mika Kuoppala <mika.kuoppala@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>