drm/i915: On GPU reset, set the HWS breadcrumb to the last seqno

After the GPU reset and we discard all of the incomplete requests, mark
the GPU as having advanced to the last_submitted_seqno (as having
completed the requests and ready for fresh work). The impact of this is
negligible, as all the requests will be considered completed by this
point, it just brings the HWS into line with expectations for external
viewers.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460010558-10705-2-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -2882,6 +2882,8 @@ static void i915_gem_reset_engine_cleanup(struct drm_i915_private *dev_priv,
 		buffer->last_retired_head = buffer->tail;
 		intel_ring_update_space(buffer);
 	}
+
+	intel_ring_init_seqno(engine, engine->last_submitted_seqno);
 }
 
 void i915_gem_reset(struct drm_device *dev)