drm/i915: Use complete virtual address range on 32-bit platforms

With the offset length being taken care of in ("drm/i915/gtt: Allow >=
4GB offsets in X86_32"), the code should be finally safe in 32-bit
kernels.

This reverts commit 501fd70f
Author: Michel Thierry <michel.thierry@intel.com>
Date:   Fri May 29 14:15:05 2015 +0100

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

Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>