drm/i915/selftests: Force a preemption hang

Inject a failure into preemption completion to pretend as if the HW
didn't successfully handle preemption and we are forced to do a reset in
the middle.

v2: Wait for preemption, to force testing with the missed preemption.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180716132154.12539-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_lrc.c

@@ -559,6 +559,9 @@ static void complete_preempt_context(struct intel_engine_execlists *execlists)
 {
 	GEM_BUG_ON(!execlists_is_active(execlists, EXECLISTS_ACTIVE_PREEMPT));
 
+	if (inject_preempt_hang(execlists))
+		return;
+
 	execlists_cancel_port_requests(execlists);
 	__unwind_incomplete_requests(container_of(execlists,
 						  struct intel_engine_cs,