drm/i915/selftests: Exercise independence of per-engine resets

If all goes well, resetting one engine should not affect the operation of
any others. So to test this, we setup a continuous stream of requests
onto to each of the "innocent" engines whilst constantly resetting our
target engine.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170721123238.16428-16-chris@chris-wilson.co.uk
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>