drm/i915/selftests: Stress resets-vs-request-priority

Watch what happens if we try to reset with a queue of requests with
varying priorities -- that may need reordering or preemption across the
reset.

v2: Tweak priorities to avoid starving the hanging thread.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180322073533.5313-2-chris@chris-wilson.co.uk


Reviewed-by: Jeff McGee <jeff.mcgee@intel.com>