drm/i915: Exercise filling and removing random ranges from the live GTT

Test the low-level i915_address_space interfaces to sanity check the
live insertion/removal of address ranges.

v2: Split out the low-level operations to a new test

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/20170213171558.20942-31-chris@chris-wilson.co.uk