drm/i915/selftests: Use sgt_iter for huge_pages_free

Use the normal sgt_iter to walk the pages scatterlist on free so that we
handle the error path correctly.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=112225


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191128232946.546831-1-chris@chris-wilson.co.uk