drm/i915: Remove incorrect restriction on 32bit offsets in ppGTT backend

This is the wrong layer to apply an arbitrary restriction and the wrong
error code (object too large!). If we do want to prevent large offsets
being return to the user on 32bit systems (to hide bugs in userspace),
you want to restrict the drm_mm range manager instead. This first tells
userspace about the correct size of the GTT they can use (so they don't
try and overallocate object or batches), and fixes the eviction logic to
avoid the eventual and *guaranteed* error.

Fixes regression in
commit d7b2633d
Author: Michel Thierry <michel.thierry@intel.com>
Date:   Wed Apr 8 12:13:34 2015 +0100

    drm/i915/gen8: Dynamic page table allocations

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