drm/i915: Include the current timeline seqno for debugging execlists

While this is mainly only useful for ELSP[0], it is definitely useful to
know the current timeline seqno wrt to the queued set of requests for
that port, as this carries additional information above and beyond the
near-defunct global_seqno and global HWSP.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190211131004.11634-1-chris@chris-wilson.co.uk