drm/i915/debugfs: Update pageflip information

Show the last submitted seqno to the engine, not the overall next seqno,
as this is more pertinent information when inspecting the pageflip and
whether the CS or display engine stalled.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20161124144750.2610-2-chris@chris-wilson.co.uk


Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>