drm/i915: Stop showing seqno info from debugfs/i915_interrupt_info

Since the seqno information shown from i915_interrupt_info is just a
small subset of i915_engine_info, remove it.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171209104418.4223-2-chris@chris-wilson.co.uk