drm/i915: Kick waiters on resetting legacy rings

For reasons unknown, interrupts following a reset do not arrive, but
this can be papered over by kicking any waiter and peeking at the
breadcrumbs following the reset.

Testcase: igt/gem_eio/reset-stress
References: https://bugs.freedesktop.org/show_bug.cgi?id=105957
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180730075351.15569-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -527,6 +527,8 @@ static int init_ring_common(struct intel_engine_cs *engine)
 	if (INTEL_GEN(dev_priv) > 2)
 		I915_WRITE_MODE(engine, _MASKED_BIT_DISABLE(STOP_RING));
 
+	/* Papering over lost _interrupts_ immediately following the restart */
+	intel_engine_wakeup(engine);
 out:
 	intel_uncore_forcewake_put(dev_priv, FORCEWAKE_ALL);