drm/i915: Include ring->emit in debugging

Include ring->emit and ring->space alongside ring->(head,tail) when
printing debug information.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180307134226.25492-4-chris@chris-wilson.co.uk