drm/i915/selftests: prefer random sizes for the huge-GTT-page smoke tests

Ditch the dubious static list of sizes to enumerate, in favour of
choosing a random size within the limits of each backing store. With
repeated CI runs this should give us a wider range of object sizes, and
in turn more page-size combinations, while using less machine time.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
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/20191025153728.23689-6-chris@chris-wilson.co.uk