drm/i915/selftests: test descending addresses

For igt_write_huge make sure the higher gtt offsets don't feel left out,
which is especially true when dealing with the 48b PPGTT, where we
timeout long before we are able exhaust the address space.

v2: just use IGT_TIMEOUT

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171123135421.17967-2-matthew.auld@intel.com