drm/i915: Make engine state pretty-printer header configurable

Pass in a format string (and args) to specify the header to be emitted
along with the engine state when pretty-printing. This allows the header
to be emitted inside the drm_printer stream, so sharing the same prefix
and output characteristics (e.g. debug level and filtering).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171208012303.25504-2-chris@chris-wilson.co.uk