drm/i915/selftests: rein in igt_write_huge

Rather than repeat the test for each engine, which takes a long time,
let's try alternating between the engines in some randomized
order.

v2: fix gen2 blunder
    fix !order blunder
    more cunning permutation construction!

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/20171123135421.17967-1-matthew.auld@intel.com