drm/i915: Live testing of lowlevel GTT operations

Directly test allocating the va range and clearing it, this bypasses the
use of i915_vma_bind() and inserting the pages to focus on testing of
the pagetables.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170213171558.20942-32-chris@chris-wilson.co.uk